"I changed the stock in Shopify, so why has Amazon not updated?" is one of the questions we are asked most often. This article explains exactly when your merchant-fulfilled (FBM) stock is pushed to Amazon, so you can tell the difference between a delay that is normal and a problem that needs fixing.
Everything here applies to FBM SKUs only, meaning stock you hold and ship yourself. FBA stock is handled differently, and deliberately so. See Inventory Sync for the distinction.
There are exactly three ways your Shopify quantity gets pushed up to Amazon. There are no others.
On the Inventory Sync page, Sync all FBM inventory pushes the current Shopify quantity for every FBM SKU the app tracks, in a single run. This is your catch-up tool: use it after a stocktake, a bulk edit, or a CSV import, or the first time you switch the feature on and want everything brought into line at once.
Each product row carries its own Sync FBM button, which pushes just that SKU. It is the right tool when you have corrected a single item, or when you are testing whether a specific product pushes cleanly.
This is the one that matters day to day, and the one most merchants do not realise is there.
The app subscribes to Shopify's inventory_levels/update webhook. Whenever a stock level changes in Shopify, for any reason at all, Shopify notifies the app immediately and the app pushes the new quantity to Amazon. That covers a sale coming through, a manual edit in the Shopify admin, a return being restocked, a bulk update, or a change made by another app or by your own integration. It all flows through the same webhook.
In practice this means the push lands within seconds, not minutes or hours.
You normally do not need to press anything
Because the webhook does the work, the two buttons are there for catch-up and for troubleshooting. In steady state, changing your stock in Shopify is enough on its own. If you find yourself pressing Sync all FBM inventory every morning out of habit, you can stop.
It is worth stating plainly, because it differs from how other parts of the app work and from how some competing apps work. Price sync runs on a schedule tied to your plan. FBM inventory sync does not. There is no hourly or six-hourly inventory job running quietly in the background.
That is not an omission. A schedule would make your Amazon stock up to several hours stale, and stale stock is how you oversell. Real time push via the webhook is both faster and safer.
The automatic push ignores any SKU the app has identified as FBA, meaning it appears in your Amazon FBA inventory.
This is intentional. Amazon owns that stock: the units are physically in Amazon's warehouses, and Amazon's own systems are the authority on how many are available. Pushing a Shopify quantity over the top of Amazon's own count would be both wrong and potentially damaging. So for FBA SKUs the app reports what Amazon holds, and writes nothing.
If you expected a SKU to be pushed and it was not, the first thing to check is whether the app is treating it as FBA. The fulfilment type is shown on every row of the Inventory Sync page.
Work through these in order. In our experience one of them is nearly always the cause.
1. Is the product published through Product Sync?
The app can only push stock into a listing it actually manages. If the product has not been published to Amazon via Product Sync, there is no listing on the app's side to update. Products that are merely mapped for price sync on the Product Mapping page appear on the Inventory Sync page under Price-mapped products (read-only), and no inventory is written for them. Publish the product first.
2. Is the SKU FBM rather than FBA?
Check the fulfilment type on the row. If it says FBA, the push is being skipped on purpose, as explained above. Manage that stock in Seller Central or through an inbound shipment.
3. Is Inventory Sync included in your plan?
Inventory Sync is a paid feature, available on Growth and above. On the Free plan the pushes do not run at all, including the automatic webhook push. If you are on Free, this alone explains it. See Plans and billing.
If all three check out and the quantity on Amazon is still wrong, press Sync FBM on that individual product and watch the result. A targeted push will surface any error Amazon returns for that specific listing, which is more informative than a bulk run.