Amazon inventory sync: the complete guide for multichannel sellers

One source of truth, two directions

Real inventory sync is bidirectional. A sale on Shopify must reduce the Amazon quantity, and an Amazon order must reduce Shopify. If either direction is missing you are running two independent stock counts that drift apart every day you trade.

The authoritative source should be the system where stock is physically managed — usually your store or an ERP. Every other channel is a follower that receives updates and never originates them.

How fast is fast enough?

Scheduled hourly syncs are the industry default and they are the reason overselling still happens: a hot SKU can sell out in minutes. Event-driven sync — a webhook fires the moment stock changes, and the update is pushed immediately — closes the window to seconds without hammering rate limits, because updates only happen when something actually changed.

Buffers, safety stock and reserved quantity

Even with fast sync, physical picking and returns create small discrepancies. A safety buffer (hold back 1–2 units, or a percentage on high-velocity SKUs) absorbs that noise. Amazon's own reserved quantity — units allocated to unshipped orders or in FC transfer — should be excluded from the available figure you publish elsewhere.

Step by step with SellerApex

1. 1

   Pick the source of truth

   Usually Shopify or your ERP; every other channel becomes a follower.

2. 2

   Enable webhooks

   Stock changes push instantly rather than waiting for a scheduled run.

3. 3

   Set safety buffers

   Hold back a small reserve on fast-moving SKUs.

4. 4

   Turn on order sync

   Amazon orders decrement source stock so the loop is closed.

5. 5

   Watch the drift report

   Any SKU where channels disagree is flagged for reconciliation.

Frequently asked questions

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