How do I return an item purchased with Apple Pay in store?
You take the item(s) back to the retailer as normal. Some retailers will be able to offer refunds back to the device from which the original payment was made, while other may have to refund the plastic card.
You may need to provide the last four digits of the Device Account Number to the retailer. This can be found by opening the Wallet app and tapping the (***) symbol in the top right corner and then scrolling down to find Device Account Number.