What on earth is the point of sending with tracking if you are going to refund when AP say it has been delivered to the buyer's address? If they don't have a secure enough mailbox how is that your fault? I presume you selected the option not to leave the parcel in a safe place if nobody was home so it should have been taken to the PO and a card left. If that is not what happened then the buyer should be taking it up with AP.