Amazon treats tax and shipping as separate, one-time refundable buckets on every order. If those amounts have already been refunded on Amazon, any new refund that includes tax or shipping again is rejected.

Why it happens

Tax and shipping on an Amazon order can each be refunded only once. Once refunded, Amazon won't accept another refund call that touches those amounts. This commonly happens when:

• A previous Shopify-side refund already included the tax or shipping portion.
• A buyer-initiated refund or A-to-z claim on Amazon refunded the tax or shipping directly.
• The order's tax / shipping was adjusted on Amazon outside the app.
• A new Shopify refund for the same order tries to refund the item again and the tax/shipping along with it.

Fix it

You need to retry the refund without the already-refunded tax or shipping component.

On the Orders page, find the row with the Failed Refund status and click on Failed Refund. In the Failed Refund dialog:

• Refund Inventory — set the quantity for the items you're refunding.
• Refund Reason (Amazon) — pick a valid Amazon reason.
• Refund Amount — enter only the item portion that hasn't been refunded yet. Don't include tax or shipping if they were already refunded on Amazon.

Click Confirm to retry.

To confirm what's already been refunded, open the order in Seller Central. The order detail page lists each refund line — including tax and shipping — that Amazon has already processed.