Here is what the Help Center says....I'd send them a copy of that...

If you mixed up two requests and sent the wrong books to two people:

• Contact both requestors.
  • Use the buttons on the active transactions for the books to let them know what happened.
  • The active transactions for books you have mailed with be on the Books I've Mailed tab, in My Account.
• You can ask the requestors to send the books to each other
  • You provide the addresses in PMs.
  • You should also offer them each a credit or postage for sending the book to the proper destination.
• When the requestors get the books they requested, they can mark them received.
  • This means the sender "breaks even" on these transaction, but each book ends up with the person who requested it.
• If the books are delayed enough to have been declared lost at PBS:
  • the requestors can still mark them received when they get them (giving you credit) from their Transaction Archives, as described in I received a book that was 'lost in the mail'.
• If either of the requestors marked either of the books received with a problem, you got credit at that time.

Explain it to her like this:

Requestor #1 and Requestor #2 each start with 1 credit. Order books. Each receives wrong book. Each pays $2.38 to send book to correct person. Each requestor has now paid 1 credit PLUS $2.38 to get correct book.

Books are supposed to cost 1 credit, NOT 1 credit PLUS postage. Therefor, the SENDERS mistake has caused each requestor to pay extra money to receive their book.

The sender received 2 credits, one for each book. However, they do not get to keep these credits, because they made a mistake. The sender refunds 1 credit to each of the requestors to cover the cost of the postage to FIX THEIR MISTAKE.