Buyer claimed item was not delivered but tracking shows it was

Hi, I sell low priced items under $10 that are flat and can be shipped in a regular envelope via first class mail. To keep costs down, I purchase letter tracking (via Pitney Bowes tracking) so at least I can watch these envelopes go through the USPS system. It’s not the same as tracking on a package but it’s close. Today I had a customer claim he didn’t get the envelope but the Lettermail tracking has it marked as delivered. Amazon issued a refund anyway. I would like to appeal this. Have any of you had this situation happen before and do you know if Amazon would protect a buyer who bought letter tracking outside of their platform? To my knowledge, you can’t buy this tracking via Amazon.