I had the same thing happen during the busy christmas weeks. I simply sent the buyer a copy of the computer print out I went to the post office and had them print for me,showing it was delivered and told them I was sorry they didnt recieve the package and if it didnt show up in a couple days to take it up with the postal carrier.
I mean in all reality what else could I do except refund for a lost item. This is why I strongly recommend on my listings that they should purchase the insurance..and for really expensive items..I either require it or buy it myself if they choose not too..Its worth the couple bucks, Just in case.