I ship a lot of books - I sell some books on Amazon, I share books back and forth with a library school friend of mine in Illinois, and I find myself frequently sending books off to friends and family around the country.
Shipping can be heinously expensive and the post office, to my knowledge, doesn't give out coupon codes, so it's rather hard to save much money there.
However, I've discovered that most people don't know about the existence of media mail.
If you're shipping something that is "media" (this includes books, CDs, DVDs, musical scores, etc), you can send it for practically nothing - far less expensive than first class.
In fact, when we moved recently, we packed up all our books, CDs, and movies and shipped them this way because you can send a 50 lb box for $21. That is a good deal.
And when I'm only sending one book? It's lovely that it only costs about $2.