A quick note before the steps: Often, nothing will be "wrong" with how discount codes work. These reports are edge cases — most customers are still redeeming codes with no friction. Use this article to identify which edge case is happening, not to assume something is broken platform-wide.

Ask the customer to confirm they entered the code at Amazon checkout, not on your Shopify store. A code generated for Amazon won't work on Shopify and vice versa — this is the most common mix-up, though less likely if the customer already told you they were checking out on Amazon.

Amazon promotions can be capped so each customer can only redeem one code per batch. If a customer already redeemed a 20%-off code from this batch previously, Amazon will block them from redeeming a second one — even if the new code is valid.

This setting isn't visible from the main Seller Central promotion screen, so you'll need to check it manually:

Watch: how to check the per-customer redemption limit (starts at 3:14)

Amazon's checkout page has a display quirk: after entering a discount code, the price shown doesn't update right away. The discount only appears once the customer clicks "Review order" further down the page. This can make it look like the code failed when it actually applied correctly.