I wonder if your post office receipt is really as valueless as you think it is. I had an Amazon chargeback once and they asked me to provide any documentation I had. All I did was submit on their system that had a post office receipt with the buyer zipcode for a shipment via media mail and the weight of the book. I didn't even have to give them the receipt (although I kept it aside thinking they'd ask). A couple months later, I went to look and it said it had been resolved in my favor.
Your receipt probably has the weight and zip as well, and might be enough for paypal to nail this guy.
Incidentally, I have had some swap books lost in the mail. It was when I had the USPS holding my mail when I was on vacation. I got most of my stuff, but two of the books never made it and had to be marked lost on the swap systems. But I can't think of a time when I've lost stuff when I've been receiving active mail. And I swap hundreds of books a year.