Gah, post office. I'm sure they have a quota for insane OCD nitpickers who missed the clue train.

Some years ago I tried to send the rulebook of a science fiction role playing game to a friend. As a letter. And got into a completely surreal debate with the post office worker that I couldn't send this as a book (which is cheaper, but what I hadn't planned to do either way), because there were adverts in it. Not adverts for other products from the same company, as you might think. No, adverts for holidays on Mars!