The most common issue for out-of-sync inventory is waiting for Amazon to catch up with updates sent by the Shopify Marketplace Connect app. We process and send inventory updates to Amazon in real time but Amazon can take some time to process them at their end.
If you have waited a few minutes then check to see if you have your Shopify inventory policy set to "Allow customers to purchase this product when it's out of stock" and the inventory setting to "Use Shopify inventory settings".
For this kind of setup, Amazon will always display the inventory quantity of 1, so you would have to rely on Shopify inventory amount for accurate numbers. Change the setting in Shopify to avoid this happening.
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