It was a different time, wasn't it?
People would laugh today if you wrote to a shop for a book without including a credit card number. And Frank sent the book to her on good faith that she would pay. He didn't seem to charge her for postage (unless it was very minimal). That is amazing to me. She requested all of her books without sending money in advance and he sent them.
It was a different world back then. I know there were fewer distractions - no TV, internet, etc., but it seems people were a lot nicer and more trusting then.