When a book goes lost, where does your credit you receive back come from?

PBS or the user who sent the book which got lost?

PBS

when you order a book the credit goes into "escrow".  PBS holds onto the credit until you mark the book received or the book goes lost.

If the sender used PBS DC then PBS refunds a credit to the requestor without docking the sender-who would have received their credit when the DC was scanned. The cost of these credits is covered by the DC fee you pay PBS-it's a small percentage of books that go lost.