Amazon requires every refund to be described at the item level — which SKU/product, how many units, and how much for each. If the refund the app submits is missing any of that detail, Amazon rejects the whole refund.

Why it happens

Shopify allows refunds that aren't tied to specific items — for example:

• A flat-dollar refund (a "goodwill" $10 refund without picking specific SKUs/products).
• A refund where line-item quantities or prices were left blank.
• A refund covering items that no longer exist on the order (e.g., you removed a line in Shopify before refunding).

When Shopify sends the app a refund without item-level breakdown, the app can't construct a valid Amazon refund — Amazon needs SKU + quantity + amount per item, every time.

Fix it

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

• Tick each SKU/product that's actually being refunded.
• Set Refund Inventory (quantity) for each one.
• Set the Refund Reason (Amazon) for each one.
• Set the Refund Amount for each ticked SKU/product so the totals add up to what you intended to refund.

Click Confirm. The app sends Amazon a per-SKU refund with the detail it needs.

Avoid it next time

Issue refunds in Shopify by selecting the items and quantities rather than entering a flat dollar amount. That way each refund carries the per-SKU detail Amazon expects.