SOLVED: SEE EDIT BELOW

I accidentally sent a customer the wrong item, and they requested a replacement through Amazon before messaging me. At the same time, they placed a second order for the correct item themselves.

To make things right, I refunded the first order and let them know there was no need to return the original item, they could keep it. Then I processed the second order, so they have received the correct item.

Now Amazon is telling them they need to return the first item or they’ll be charged for it using the credit card on file. Since I don’t actually need the first item back, how can I resolve this with Amazon? When I issued the refund, there wasn’t an option like "Refund without return."

Has anyone dealt with this before? I just want to make sure my customer isn’t charged unnecessarily.

Thanks for any advice!

EDIT: I discovered I have this wrong. I had no idea the 2nd order was actually a replacement order. This was confusing to me as it just came through shipstation as a regular order and I never looked at the $0 total. So Amazon allowed me to send a replacement without first receiving the return, but is expecting to see a return scan or they will charge the customer. Looks like Amazon is doing something right for the seller at least. I suppose if they now send me an empty package I'll be out because of the original refund (which is my fault for not understanding what happened).