I had marked that I could mail the books, but I had not printed the wrapper yet. If I cancel, my books go to the back of the FIFO line right? What does she need to do to cancel?

If you haven't printed the wrapper, then she needs to cancel on her end.  You will retain your place in the FIFO line.

K, thanks. She got it figured out and canceled.

Just so you know for the future, no you wouldn't have kept your place in the FIFO, and she would have still had the books on order if you had cancelled. Her order would have gone to the next copies of the books in the FIFO, and would NOT have been cancelled.