Try posting them in a different Posting box. We had an issue like this where mail posted in a certain Posting box, never made it to it's destination, commented on by several neighbours. The Postal worker emptying it was seen rifling though the mail. Used another Posting box and no problems. Occasionally our Postie will put the neighbour's mail in our mailbox and vice versa. In your case, it could be buyers being impatient and claiming not received before allowing sufficient time. I make all sales (parcels) signature on delivery now, since I discovered that Australia Post will take no responsibility once the parcel is "safe dropped" even though on some occasions it is not safe to do so. With many deliveries being late at the moment, it is safer to register letters as well. Let the buyer pay for it. Then you will have peace of mind.