Amalia E. Barrios

I sold a comic to a buyer and shipped the item on Jul 9 and it was delivered on Jul 10 via USPS First Class mail. The tracking info was entered into the AA Sales Detail order info. The buyer has now contacted me twice that he has not received the comic. Since I have proof of sending, along with the USPS tracking info saying it was delivered to the address on file, am I obligated to refund the buyer?