SERVICES · RELAY MODULES
Inventory replenishment service for Amazon sellers who also sell on Walmart and Shopify
Stock Flow keeps one stock count across Amazon FBA, Walmart WFS, Shopify and your 3PL, sets reorder points from your real sales and lead times and drafts every purchase order for your approval.
One stock number
1,284 units, SKU-2041
Reorder point crossed · PO draft ready
An inventory replenishment service decides when and how much to reorder, then tracks each order until it is ready to sell. Stock Flow (RELAY-03) builds this in your accounts for Amazon sellers who also sell on Walmart and Shopify: one stock count across FBA, WFS, Shopify and your 3PL, reorder points you approve, purchase order drafts and stockout alerts. It is an advanced Relay Module, $1,500 to $3,500 fixed in writing, and it is included in Ops Desk Growth and Scale.
Why reorders slip when you sell in three places
Reorders slip because nobody owns one number. Amazon shows what is at FBA, Walmart shows WFS, the 3PL portal shows the warehouse and the purchase order sheet shows what is on the water, each a little out of date. A reorder happens when someone notices, usually after the best seller has gone out of stock on the channel that matters most.
A better spreadsheet will not fix it. What works is one count, one set of reorder rules and one person who reviews them with you every week. For the diagnosis and the cost of a stockout, see stockouts on best sellers.
What the inventory replenishment service gives Amazon sellers
-
One stock count by SKU and location:
FBA, AWD if you use it, WFS, your 3PL and stock in transit, from one master SKU map.
-
A weekly reorder plan:
SKU, days of cover, reorder point and suggested quantity, grouped by supplier so one supplier gets one order.
-
Purchase order drafts
with supplier, SKU, quantity, unit cost, target ship date, destination split (FBA, WFS, 3PL) and the week the payment hits your cash.
-
Alerts:
a top SKU heading below its cover target, an inbound shipment running late, a sync error between systems.
The master SKU map ties every ASIN, Walmart item ID and Shopify variant to one physical SKU, including bundles and multipacks, so a two-pack sold on Walmart takes two units from the same count Amazon uses.
Where each unit should go: the destination rules
Each purchase order draft splits the quantity by destination, because FBA, WFS and your 3PL each charge for too little or too much stock in different ways. Checked Oct 20, 2026.
| Destination | Rule the plan respects | Source |
|---|---|---|
| Amazon FBA | The low-inventory-level fee applies only when both the 30-day and the 90-day historical days of supply are below 28 days, so the plan keeps FBA cover above 28 days where the economics support it. Amazon adjusted the fee on Jan 15, 2026; we confirm current exemptions in your account | Amazon Seller Forums announcements (2024; Oct 15, 2025) |
| Amazon FBA | Storage costs rise with age and in October to December; current rates are read in Seller Central, not printed here | Amazon |
| Amazon FBA | US FBA prep and labeling services ended Jan 1, 2026, so prep and labels are planned at your supplier or 3PL and counted in lead time | Amazon SP-API changelog |
| Walmart WFS | Storage is $0.75 per cubic foot a month, plus $1.50 per cubic foot from October to December for units stored more than 30 days; long-term storage is $2.25 per cubic foot a month for days 366 to 450 and $7.50 after 450 days, from Jun 30, 2026 | Walmart WFS fees (May 27, 2026) |
| Your 3PL | Stock shared by Shopify and seller-fulfilled Walmart orders keeps a buffer for the channel with the stricter cancellation standard | Our inventory and 3PL method |
The reorder formula, safety stock methods and a worked example are on our inventory and 3PL page. For fee detail, see the Amazon low-inventory-level fee and WFS fees guides.
The inputs you choose
-
Sales window per SKU group:
30, 60 or 90 days. A steady seller uses 90; a new or seasonal SKU uses 30.
-
Lead time per supplier,
counted from order to available-to-sell, not to dock: production, freight, customs, prep and receiving at FBA, WFS or your 3PL.
-
Safety stock method per group,
from a fixed number of days to a method based on how much demand and lead time vary.
-
Cover targets per destination.
-
Supplier minimums and case packs,
so a suggested 1,130 units becomes the 1,152 your supplier will actually ship (illustrative).
Replenishment is only as good as its inputs, and you set each one in the spec:
What it connects to on Amazon, Walmart, Shopify and your 3PL
Stock Flow reads counts and inbound status through official interfaces, under named-user access.
- Amazon:
the FBA Inventory API and inventory ledger reports for counts; the inventory planning and restock recommendation reports as a cross-check against Amazon's own suggestions; the Fulfillment Inbound API for shipment status; the AWD API if you store at AWD; FBA_INVENTORY_AVAILABILITY_CHANGES notifications for quantity changes between refreshes.
- Walmart:
the Inventory API and the WFS inventory and inbound shipment APIs under Fulfillment Management; the Inventory OOS event notification.
- Shopify:
inventory levels by location, with the Shopify Flow trigger "Product variant inventory quantity changed" for low-stock alerts.
- Your 3PL:
on-hand, receiving and advance ship notices. Many warehouses send these as EDI (the 856 advance ship notice and 945 shipping advice); others offer an API, such as ShipBob's or Extensiv's REST API, or a scheduled export.
- Your purchase orders,
and the inventory or order system you already use.
What is automated and what a person does
Automated:
counts, days of cover, reorder triggers, purchase order drafts and alerts.
A person:
the weekly reorder review with you, supplier follow-up, inbound tracking to arrival and exceptions such as a short shipment or a stranded FBA unit. On Ops Desk, that person is your named ops lead. On a module-only build, your team runs the review with the playbook we hand over.
Stock Flow drafts the purchase order. You approve it. We never commit your money to a supplier, carrier or 3PL without your written approval. Each draft shows the week its deposit and balance hit your cash, and if you also run Profit and Cash Pulse those payments appear in your 13-week view.
How the build runs and how you accept it
Week 1 is the spec: SKUs, locations, lead times, safety stock method, alert thresholds and your PO template. Weeks 1 to 2 connect the systems and build the master SKU map. Weeks 2 to 3 build the reorder logic and PO drafts. Weeks 3 to 4 run a test day and acceptance.
The acceptance tests in a typical Stock Flow spec:
- Counts by location reconcile to each channel's report on the same day.
- Every active SKU has a lead time and a reorder point, or a named exception.
- A simulated sales spike on a test SKU raises an alert at the set threshold.
- A PO draft shows the destination split and the cash week.
- Every open inbound shipment shows a status.
- Documentation is stored in your workspace.
After go-live, your weekly review tracks stockout days on top SKUs, days of cover by destination, late inbound shipments and aged units at WFS or FBA. We report these measures. We do not promise them.
Price, timing and what you own
Stock Flow is an advanced module: $1,500 to $3,500, fixed in writing before the build, built in 1 to 4 weeks. You pay 50% at the start and 50% at acceptance; anything that fails acceptance is fixed free for 60 days. It is included in Ops Desk Growth ($2,499 a month) and Scale (from $4,499 a month). One stock count across channels is also part of the 14-Day Ops Foundation (from $1,500 for one channel, $2,500 multichannel). Module Care is included on any monthly plan, or 10% of the build cost a year otherwise. Full terms are on pricing.
Founding offer: 20% off the module build, plus 20% off monthly plans for the first 6 months, for our first 25 clients or until Jan 31, 2027, whichever comes first.
The SKU map, rules and templates stay in your accounts. Named user access with multi-factor authentication. Never your owner password. No bank, payout or legal-entity permissions.
Stock Flow is not warehouse space, pick-and-pack labor or a buying service: you buy the stock and pay suppliers directly. If the same stock oversells across channels, read overselling across channels; if the 3PL's count is the problem, pair it with 3PL Audit. All ten modules are on the Relay Modules hub.
How much does Stock Flow cost?
It costs $1,500 to $3,500 to build, fixed in writing before we start, depending on how many locations, suppliers and systems it connects. It is included in Ops Desk Growth at $2,499 a month and Scale from $4,499 a month, where your ops lead also runs the weekly reorder review with you.
Do you place orders with our suppliers?
We draft every purchase order and you approve each one in writing. Once approved, we can send it, confirm the ship date and track it to arrival. We never commit your money to a supplier, carrier or 3PL without that approval, and you pay suppliers directly.
Do we need new inventory software?
Usually not. Stock Flow is built in the tools you already use where they can hold one count. If they cannot, we recommend options in writing and you subscribe directly, in your name. We do not resell or mark up software.
Does it include Amazon AWD?
Yes, if you use it. AWD appears as its own location in the count and the plan, read through Amazon's AWD API, so transfers from AWD to FBA show as stock on the way rather than as missing units. The reorder plan then decides how much goes to AWD for bulk storage and how much ships straight to FBA.
How do you set safety stock?
Per SKU group, using a method you approve in the spec, from a fixed number of days of cover to a calculation based on how much your demand and lead times vary. The formulas and a worked example are on our inventory and 3PL page.
How fast can it be live?
One to four weeks from working access and an approved spec. If you start with the 14-Day Ops Foundation, one stock count across channels is live in 14 business days from full access, or we keep working at no charge until it is.
A free Profit Leak Scan includes an inventory risk section that prices stockout and storage exposure, so you can see what Stock Flow would fix first.
Ecomrelays LLC is a Texas company. Part of our team works outside the US, as one managed team with a named US-facing lead. 597 Mountcastle Dr, Rockwall, TX 75087. +1 (469) 331-3129.
Sources12 sources
- 01marketplacelearn.walmart.comWFS storage $0.75 per cubic foot a month; long-term storage $2.25 for days 366 to 450 and $7.50 over 450 days from Jun 30, 2026; updated May 27, 2026; checked Oct 20, 2026Primary
- 02developer-docs.amazon.comUS FBA prep and labeling services ended Jan 1, 2026; checked Oct 20, 2026Primary
- 03developer-docs.amazon.comFBA inventory ledger, inventory planning and restock recommendation reports; checked Oct 20, 2026Primary
- 04developer-docs.amazon.comFBA Inventory API, Fulfillment Inbound API v2024-03-20, AWD API v2024-05-09; checked Oct 20, 2026Primary
- 05developer-docs.amazon.comFBA_INVENTORY_AVAILABILITY_CHANGES; checked Oct 20, 2026Primary
- 06developer.walmart.comWalmart Inventory and Fulfillment Management APIs; Inventory OOS event; checked Oct 20, 2026Primary
- 07help.shopify.comShopify Flow trigger "Product variant inventory quantity changed"; checked Oct 20, 2026Primary
- 081edisource.comEDI 940, 945 and 856 in warehouse communication; checked Oct 20, 2026Primary
- 09developer.shipbob.comShipBob API guides for orders, inventory and receiving; checked Oct 20, 2026Primary
- 10help.extensiv.comExtensiv 3PL Warehouse Manager REST API for orders and inventory; checked Oct 20, 2026Primary
- 11sellercentral.amazon.comAmazon announcement, News_Amazon: the low-inventory-level fee applies to units shipped with historical days of supply below 28 days, and is avoided if either the 30-day or the 90-day figure is above 28 days; 2024; checked Oct 20, 2026Primary
- 12sellersasksellers.comAmazon's 2026 fee announcement text, posted Oct 15, 2025: low-inventory-level fees adjusted from Jan 15, 2026; checked Oct 20, 2026. Amazon storage rates are not printed on this page because they could not be verified on an Amazon page today.Primary
Next step
Ask a real expert. Free.
A real person replies within one business hour, 9am to 6pm US Central, Monday to Friday.