I bought a box of books at a sale and in the box was a family bible with an inscription. I searched the Ancestory site and found a member of the family looking for info on her relatives. I emailed her and offered to send her the book. She sent me her address and off the book went. She emailed me and said thanks. She did not offer to pay the $10.00 it cost for postage, not that I would have accepted anything if she would have offered. I was doing a PIF because I wanted to.
Several months ago I was going to make a pattern from a magazine and I discovered that some of the pages were missing. A wonderful lady had two copies of the magazine and sent me one. I offered to pay her and she said Pay it Forward. I have done it before and I will do it again, but I do wish that the book lady would not have broken the chain of PIF.