INVENTORY AND 3PL · STOCK FLOW · 3PL AUDIT
Inventory management services for ecommerce brands, with a monthly 3PL audit
We keep one stock count across Amazon FBA, Walmart WFS, Shopify and your 3PL, set reorder points from your real sales and lead times and check every 3PL invoice against what actually happened in the warehouse.
3PL Audit $750 · Stock Flow $1,500 to $3,500 · both run inside Ops Desk Growth at $2,499 a month
- Weekly Reorder plan updated; POs drafted for your approval; days of cover reviewed against fee thresholds; buffer review
- Monthly 3PL invoice audit; 3PL and store stock reconciliation; aged stock ranked by age and sales speed; reimbursement claims filed
- Quarterly Inbound plan for peak season; removal and liquidation decisions; 3PL performance review
Quick answer
Ecommerce inventory management services keep the right stock in the right place without overselling or paying avoidable fees. Ecom Relays connects Amazon FBA, Walmart WFS, Shopify and your 3PL into one stock count (Stock Flow), sets reorder points you approve and audits 3PL invoices and stock counts every month (3PL Audit). From $750, or included in Ops Desk Growth.
What our inventory management services cover
The service has four parts. You can buy the two automations on their own, or have them run every day inside Ops Desk.
| Part | What it does | How you can buy it |
|---|---|---|
| Stock Flow (RELAY-03) | One inventory truth across Amazon FBA, Walmart WFS, Shopify and your 3PL; reorder points; purchase order drafts; stockout alerts | Built once for $1,500 to $3,500, or included in Ops Desk Growth and Scale |
| 3PL Audit (RELAY-08) | Monthly invoice checks against your contract and warehouse data; stock reconciliation between the 3PL and your store; delivery-time monitoring | Built once for $750, or monthly 3PL invoice checks included in Ops Desk Growth and Scale |
| Daily inventory operations | Stock and inbound checked every business day, POs drafted for your approval, inbound shipments tracked to arrival | Ops Desk, from $1,499 a month |
| One-time setup | One stock count across channels and the 3PL, as part of connecting your operations | 14-Day Ops Foundation, $1,500 or $2,500 |
We do not run warehouses, resell software or hold your stock. Everything we build lives in your accounts and the tools you subscribe to.
How do you sync inventory across Amazon, Walmart and Shopify without overselling?
You keep one source of truth, map every listing to the right physical SKU and hold a buffer on the channel with the strictest cancellation rules. Overselling happens when one physical unit is offered on two channels at once and both sell it.
The causes are usually mundane. One inventory vendor lists sync delays, no single stock count, canceled orders and returns that never flow back into inventory, "continue selling when out of stock" left on in Shopify and bundles that keep their own count instead of drawing from their components (vendor view: Sumtracker, Oct 15, 2026). In our experience the other repeat offender is FBA or WFS stock that is not mapped to the right location, so a channel sells units it cannot reach.
It matters because cancellations are a scored metric.
- Walmart expects cancellations at 2% or below. Walmart measures its standards over the last 30 or 60 days and says missing them can lead to suppression, suspension or termination (Walmart Marketplace Learn, updated Jul 28, 2026). On 3,000 orders, 61 cancellations take you over the line.
- Amazon expects a pre-fulfillment cancellation rate under 2.5% (Sell on Amazon, Oct 16, 2024).
What we set up with Stock Flow:
- One source of truth. The inventory or order system you already use, or one we recommend that you subscribe to directly. We do not resell software.
- A SKU, bundle and location map. Every listing on every channel points to the right physical SKU, every bundle subtracts its components and every warehouse location is named.
- Sync frequency and buffers per channel. Stock you share between channels gets a buffer on the channel with the strictest cancellation rules, so a late sync does not become a cancellation.
- Channel rules. Stock at Amazon FBA and Walmart WFS is counted where it sits. If you fulfill Shopify or Walmart orders from FBA through Multi-Channel Fulfillment, that stock is shared and buffered like any other.
- A test day. Before we call it live, counts on every channel reconcile against the source on the same day.
How buffers work on shared stock (Illustrative · sample figures):
| Channel | Stock pool | Buffer | Shown as available |
|---|---|---|---|
| Shopify | 500 units at your 3PL | 5% (25 units) | 475 |
| Walmart, fulfilled by you from the same 3PL | Same 500 units | 10% (50 units) | 450 |
| Amazon FBA | 600 units at Amazon | None; Amazon counts its own stock | As Amazon reports it |
The buffers are rules you approve. We review them when sales speed changes or a channel's metrics move. The multichannel inventory sync guide walks through the method in more depth, and overselling across channels covers the problem from the symptom side.
Received is not the same as put away
A unit that has arrived at a warehouse is not yet a unit you can sell. A shipment goes through several states inside a warehouse: arrived at the dock, received and counted against the advance ship notice (ASN), put away into a pick location and available to sell.
If your store treats "received" as "available", it sells units the warehouse cannot pick yet. The order then ships late or gets canceled. The same gap exists at Amazon: units you shipped are not available until Amazon finishes checking them in.
What we do about it:
- Count stock as available only once it is put away, using the status your 3PL reports.
- Check every ASN (by EDI 856 or your 3PL's portal) against the purchase order, and raise short or over shipments inside your 3PL's claim window.
- Track dock-to-stock time per 3PL, so you know how many days to add to lead time.
How we set reorder points: the math
A reorder point is the stock level at which you place the next purchase order so new stock arrives before you run out.
Reorder point = average daily sales × lead time in days + safety stock
- Lead time is the whole path, not just the supplier: production, freight, customs and receiving and put away at FBA, WFS or your 3PL.
- Safety stock covers the days when sales run fast or shipments run late. A simple, conservative method: (highest daily sales × longest lead time) minus (average daily sales × average lead time). For SKUs with steady demand we use a service-level method instead: a factor for the in-stock level you choose × the standard deviation of daily sales × the square root of lead time.
- Days of cover = (available stock + inbound stock) ÷ average daily sales.
Worked example (Illustrative · sample figures):
| Input | Value |
|---|---|
| Average daily sales, all channels | 40 units |
| Highest daily sales in the last 90 days | 55 units |
| Average lead time: 30 days production, 21 days freight and customs, 7 days receiving and put away | 58 days |
| Longest lead time seen | 68 days |
| Demand over the average lead time: 40 × 58 | 2,320 units |
| Safety stock: (55 × 68) minus (40 × 58) = 3,740 minus 2,320 | 1,420 units |
| Reorder point: 2,320 + 1,420 | 3,740 units, counting stock on hand and inbound |
| Amazon floor, if 25 of the 40 daily units sell through FBA: 25 × 28 days | Keep at least 700 units available at Amazon |
Try your own numbers in the reorder point calculator.
Then we check the plan against the fee rules that punish too little and too much stock.
| Rule | Threshold | Source, date |
|---|---|---|
| Amazon low-inventory-level fee | Charged only when both 30-day and 90-day historical days of supply are below 28 days; exemptions apply | Amazon Seller Central fee rules, as checked Oct 20, 2026 |
| Amazon monthly storage, standard size | Higher in October to December than the rest of the year; we use the current rates in your Seller Central account | Amazon Seller Central |
| Amazon holiday peak fee | About $0.32 a unit on average, Oct 15, 2026 to Jan 14, 2027 | Supply Chain Dive (Jul 13, 2026) |
| Amazon prep and labels | Amazon stopped offering FBA prep and item labeling services in the US on Jan 1, 2026; unprepped units that arrive damaged or get lost are not eligible for reimbursement | Packaging Insights (Oct 29, 2025) |
| Amazon lost or damaged inventory claims | Claims for units lost or damaged in fulfillment centers are due within 60 days of the item being reported lost or damaged, and are reimbursed at manufacturing cost (since Mar 31, 2025) | eComEngine (vendor); Amazon Seller Forums |
| Walmart WFS long-term storage | $2.25 per cubic foot a month for days 366 to 450; $7.50 over 450 days, from Jun 30, 2026 | Walmart WFS fees (updated May 27, 2026) |
Fee rules change often. We recheck this table against each platform every month; the Amazon low-inventory-level fee guide, FBA fees in 2026 and WFS fees carry the detail, and the WFS storage fee calculator prices aged stock.
Why it matters: in a 2002 global study of retail out-of-stocks, 31% of shoppers facing an out-of-stock bought elsewhere, 26% switched brand and 9% bought nothing (FMI and GMA, Oct 14, 2002). Too much stock costs too: storage that steps up with age and season.
We review reorder points weekly, put slow SKUs on their own rule (buy to sales speed, not to the supplier's minimum order) and rank stock by age and sales speed every month so slow stock is moved, bundled, promoted or removed before storage charges step up. You approve every purchase order. If top sellers keep running dry, start with stockouts on best sellers.
3PL invoice audit: what we check every month
A 3PL invoice audit is a line-by-line check of your warehouse bill against your contract and the warehouse's own data. A 3PL invoice is built from rate cards, activity counts and pass-through carrier charges. Errors tend to repeat every month until someone checks, because they usually start in the setup: an old rate card, a wrong storage basis, a credit never applied.
This is the checklist we run (sources: InvoiceDataExtraction buyer's guide, updated Mar 23, 2026; Racklify, viewed Oct 20, 2026; Parabola glossary, viewed Oct 20, 2026).
| # | Check | What the error looks like | What we compare against |
|---|---|---|---|
| 1 | Rate card version | Old rates still applied after a renegotiation | Signed rate sheet and every amendment |
| 2 | Client rate swap | Another client's rates on your invoice in a shared warehouse | Your rate card |
| 3 | Storage basis | Billed on peak instead of average occupancy, or pallet and cubic-foot rounding not in the contract | Daily inventory snapshots and the contract's storage clause |
| 4 | Long-term storage aging | Wrong anniversary dates, or aging charges on units already shipped | Inventory age report |
| 5 | Receiving | Units, cartons or pallets billed above what was received | Receiving report and ASN |
| 6 | Pick and pack | Per-unit charges where the contract says per order, or extra-item fees miscounted | Order and unit counts from your store |
| 7 | Duplicate charges | The same service billed twice across periods or next to a credit memo | Prior invoices and credit memos |
| 8 | Value-added services and accessorials | Kitting, labeling or special handling with no work order behind it | Approved work orders |
| 9 | Carrier charges | Wrong dimensional weight, markup above the contract, address-correction or residential surcharges | Carrier invoices, package dimensions, contract markup |
| 10 | Credits not applied | Damage, SLA misses or returns credits that never appear | Incident and claims log |
| 11 | Minimums and account fees | Monthly minimums or system fees not in the contract | Contract |
| 12 | Stock count gap | The 3PL's count and your store's count disagree | Cycle counts and store inventory |
How a dispute works:
- We document each line: what was invoiced, what it should have been, the difference and the evidence.
- We send it in writing inside your contract's dispute window. One 2026 buyer's guide puts common windows at 30 to 90 days from invoice receipt (InvoiceDataExtraction, updated Mar 23, 2026); your contract sets yours.
- We log every dispute until it is resolved and ask for a root-cause fix when an error repeats.
- You approve anything that changes your contract or your relationship with the 3PL.
Every month we also track your 3PL's performance against its SLA: dock-to-stock time, order cycle time, on-time shipping and order accuracy. When orders keep shipping wrong, 3PL fulfillment errors explains where to look first. The 3PL invoice audit guide has the full method.
What runs daily, weekly and monthly
On Ops Desk Growth and Scale, inventory and 3PL work runs to a fixed cadence, and each cadence leaves a written record.
| Cadence | Inventory and 3PL work |
|---|---|
| Daily | Stock and inbound checked; stockout alerts on top SKUs; sync errors cleared |
| Weekly | Reorder plan updated; POs drafted for your approval; days of cover reviewed against fee thresholds; buffer review |
| Monthly | 3PL invoice audit; 3PL and store stock reconciliation; aged stock ranked by age and sales speed; reimbursement claims filed |
| Quarterly | Inbound plan for peak season; removal and liquidation decisions; 3PL performance review |
How much do inventory management services cost?
A 3PL Audit build costs $750 and a Stock Flow build $1,500 to $3,500, fixed in writing before we start. Both run every day inside Ops Desk Growth at $2,499 a month.
| Option | Price | What you get | Timing |
|---|---|---|---|
| 3PL Audit (standard Relay Module) | $750 one-time build | Invoice checks against your contract, stock reconciliation and delivery-time monitoring, built and documented in your accounts | 1 to 4 weeks |
| Stock Flow (advanced Relay Module) | $1,500 to $3,500 one-time, fixed in writing before the build | One stock count across channels and the 3PL, reorder points, PO drafts and stockout alerts | 1 to 4 weeks |
| Module Care | Included on any monthly plan; otherwise 10% of the build cost a year | Monitoring, small fixes and platform changes | Ongoing |
| 14-Day Ops Foundation | $1,500 one channel, $2,500 multichannel | One stock count across channels and the 3PL as part of the full setup | 14 business days from full access |
| Ops Desk Growth | $2,499 a month | Stock Flow and Margin Guard included, support and 3PL coordination, monthly 3PL invoice checks, daily inventory operations | Monthly; 3-month first term, then month to month |
| ERP, WMS or 3PL integrations | Scoped on a call and quoted in writing | Integration-grade builds | Per written scope |
Modules are billed 50% at the start and 50% at acceptance. Every module is built to a written spec, and anything that fails acceptance is fixed free for 60 days. Relay Module fees are fixed and published, never a percentage of your sales or ad spend. Software subscriptions are paid by you directly. Prices exclude sales tax where it applies. Every price is on Pricing.
Founding offer: 20% off launch fees, add-ons, the Foundation and Relay Module builds, plus 20% off monthly plans for the first 6 months, for our first 25 clients or until Jan 31, 2027, whichever comes first.
What you approve, and what stays yours
We draft and recommend; you approve anything that commits money or changes a contract.
- You approve every purchase order, buffer rule and change to your 3PL contract.
- We never place an order with a supplier, change a carrier or switch a 3PL without your written approval.
- Every workflow, sync rule, dashboard and checklist lives in your accounts and stays there if you leave.
- We use named user access with multi-factor authentication. Never your owner password. No bank or payout permissions.
Who this is for
This is for brands selling from shared stock across more than one channel or warehouse.
Right for you if:
- You sell on more than one of Amazon, Walmart and Shopify from shared stock, or plan to.
- You use a 3PL, or Amazon FBA or Walmart WFS, alongside your own stock.
- You have run out of top sellers, oversold or paid aged or low-inventory fees in the last year.
- Nobody has checked your 3PL invoices line by line against the contract.
Not the right fit if you want warehouse space, pick-and-pack labor or a software license. We work with your 3PL and your tools; we do not replace them. Channel-specific detail is on Amazon inventory and fees and WFS operations. Margin and landed cost sit on profit and cash reporting.
What you can see before you buy
You see the spec, the samples and the terms before you pay for a build.
- The written spec for your Stock Flow or 3PL Audit build, with acceptance tests, approved by you before work starts.
- A sample 3PL audit summary and a sample reorder report, labeled as samples.
- The 3PL billing error checklist on this page, free to use whether or not you hire us.
- Published prices, 60 days of free fixes after acceptance and Module Care terms.
- Ecomrelays LLC, 597 Mountcastle Dr, Rockwall, TX 75087. +1 (469) 331-3129.
How much do inventory management services cost?
A 3PL Audit build is $750 and a Stock Flow build is $1,500 to $3,500, fixed in writing before we start. Both are included in Ops Desk Growth at $2,499 a month with daily inventory work. Upkeep is included on any monthly plan, or 10% of the build cost a year otherwise. Software subscriptions are paid by you directly.
What is a 3PL invoice audit?
A line-by-line check of your 3PL's invoice against your contract and the warehouse's own data: rate card, storage basis, receiving, pick and pack, value-added services, carrier charges, credits and minimums. We also reconcile the 3PL's stock count with your store's, raise disputes in writing inside your contract's window and track each one until it is resolved.
What are the most common 3PL billing errors?
Old rates after a renegotiation, storage billed on the wrong basis, aging charges on units already shipped, receiving billed above what arrived, per-unit charges where the contract says per order, duplicate charges, services with no work order, carrier markups above the contract and credits that are never applied. Our 12-point checklist above covers each one.
How do you sync inventory across Amazon, Walmart and Shopify?
One source of truth in a system you own, a map from every listing to the right SKU, bundle and location, sync frequency and buffers per channel and a test day on which every channel's count reconciles. Stock at FBA and WFS is counted where it sits; shared stock gets a buffer on the channel with the strictest cancellation rules.
Do we need new software?
Usually not. We build on the inventory or order system you already use. If your current tools cannot hold one stock count across channels, we recommend options and you subscribe directly, so the license is yours. We do not resell software or take a margin on it.
How do you calculate reorder points?
Average daily sales times lead time in days, plus safety stock. Lead time covers production, freight, customs and receiving at FBA, WFS or your 3PL. Safety stock covers fast weeks and late shipments. We then check the plan against Amazon's 28-day low-inventory threshold and the storage fees that rise with age and season.
How do you help avoid Amazon's low-inventory-level fee?
We keep each item's days of supply at Amazon above the 28-day threshold where the economics make sense, plan inbound shipments early enough to cover receiving time and flag items close to the line every week. Exemptions apply to some items. We also watch the other side: aged inventory and fourth-quarter storage costs.
Will you place purchase orders for us?
We draft them from sales speed, lead time and your cash plan. You approve every one. We never commit your money to a supplier, carrier or 3PL without your written approval. Once approved, we track each purchase order and inbound shipment to arrival.
Can you work with our current 3PL?
Yes. We work with your 3PL rather than replacing it. We connect its data to your stores, audit its invoices every month, track its performance against the SLA and coordinate with your account manager there. If you decide to change 3PLs, we help compare options; the decision is yours.
How quickly can this be live?
A Stock Flow or 3PL Audit build takes 1 to 4 weeks from access and a signed spec. As part of the 14-Day Ops Foundation, one stock count across channels and the 3PL is live in 14 business days from full access, or we keep working at no charge until it is.
Sources11 sources
- 01marketplacelearn.walmart.comchecked Oct 20, 2026Primary
- 02sell.amazon.comchecked Oct 20, 2026Primary
- 03sumtracker.comvendor, published Oct 15, 2026; checked Oct 20, 2026Vendor
- 04supplychaindive.comchecked Oct 20, 2026Primary
- 05packaginginsights.comchecked Oct 20, 2026Primary
- 06ecomengine.comvendor, checked Oct 20, 2026Vendor
- 07marketplacelearn.walmart.comupdated May 27, 2026; checked Oct 20, 2026Primary
- 08fmi.orgchecked Oct 20, 2026Primary
- 09invoicedataextraction.comupdated Mar 23, 2026; checked Oct 20, 2026Primary
- 10racklify.comchecked Oct 20, 2026Primary
- 11parabola.iochecked Oct 20, 2026Primary
Next step
Find out what stockouts, overselling and 3PL errors are costing you.
The free Profit Leak Scan covers inventory, fees and claims: days of cover, stockouts, overstock, aged and low-inventory fees, plus claims you may be owed. Written report within 48 hours of working read-only access. No sales call required.
A real person replies within one business hour, 9am to 6pm US Central, Monday to Friday.