Buyer claimed a full refund only a week after ordering?

Hi! I send my items out untracked through the Royal Mail - I had a buyer manage to get a refund from Amazon after just a week (a package sent 2nd class).

They messaged me asking where their item was, I replied very quickly asking for their address to double check it was sent to the correct place (as I sometimes get buyers give me the wrong address). They never responded to me and got a full refund 2 days later from Amazon (which I’m willing to provide if an item has gone missing anyway).

Am I correct in thinking that in order for a buyer to open a A-Z they need to message the seller and wait 48 hours for a response? And can only open a a-z if the buyer has been unhelpful?

I have evidence that this was sent on time, but last time I had something like this, Amazon didn’t remove the defect from my account. I tried appealing saying that the delivery company was at fault, the buyer could keep the money and I’d just like the defect removed, which they’ve done sometimes but not the last time.

I feel like this buyer probably did get this item but knew exactly how to get a refund… How can I best appeal these? I’m able to claim the money back from the delivery company but it’s the defect I’m most worried about. Thanks for any help!