I once shipped a package through USPS (It was intake piping) and it took the seller WEEKS to recieve it. He waited about two weeks before he said anything to me, then after talking to him through PM's for about two more weeks i finally was going to refund him, but the day before i went to the bank to deposit the money to send it back he recieved the item.
If your buyer is a cool guy, ask if they don't mind waiting 2 more weeks or so just incase. Then if its not there by set date, refund.