If you did get delivery confirmation, and the post office screws up, you're clear. I'm pretty sure you only need proof that you sent it, it's out of your hands after that. Had something like that happen to me once, sold an item to someone in Canada, and didn't get delivery confirmation. He claimed that he didn't receive it, filed a dispute, and I had to give him his money back ($100, which was a huge amount of money for me at the time). Two days later, he emails me and said he got it, apparently it had been stuck in customs for a month. I asked if he can send the money back...he said he would, but I never heard anything from him since.