Long story short: A member ordered four books from me and I mailed them. They were marked 'lost' in the mail and later returned to me. A sticker from the post office stated that they had never been claimed. I PM'd the other member, but her account was on hold and she never replied. 

I eventually contacted PBS and asked if the lost books in my transaction archive could be changed (as they really weren't lost).

The PBS team changed the books and they don't show as 'lost' now. Yay!

But, I've received 4 credits for them. Why? And did the credits come from the other member's account or from PBS? I tried to PM her to ask, but now her account is showing as closed or suspended.

I don't feel right receiving 4 credits for books that I still have -- I wasn't charged extra for their return. The postage to send them was maybe worth 2 credits.

PBS will take credits from members who fail to follow the terms of service.  

What Emily said, that was also what I thought might have happened.

You lucked out too, b/c Media Mail does not include return postage and usually arrives (back to the sender, or forwarded to a new address) with postage due.

I wouldn't feel guilty. You will have to spend more postage to send them out again.  Probably more than the first time since it's unlikely that you will find another person that will request all 4. So think of those 2 bonus credits as compensation for whatever postage increase you might have to mail them out to other people.

If you're uncomfortable getting the credits you can always donate them to one of the PBS programs (schools, military,etc.).  Win/win!

Or donate them right back to PBS.  I've done that before.