Hi Angie,
I’m not really answering your question, but I wanted to add a couple of other options besides Amazon, just in case you aren’t familiar with them.
The first is Paperback Swap (www.paperbackswap.com). I recently joined this site, and I’ve already saved so much money. Basically, you post books that you want to get rid of or “swap.” Other members also post books. You can browse books by title or subject by doing a search. If someone wants a book that you’ve posted, you pay to ship it to them. Then, when you want a book, that swapper pays to ship it to you. There’s no cost for the books themselves! The books are literally free, and the only cost is the shipping. I LOVE this site!
The second site I wanted to share is http://www.half.com. This site is similar to Amazon, but different because it automatically brings up the book that is cheapest as the best fit for you. You still pay for the book, and you still pay shipping ($3.79, I think).
I always check Paperback Swap first. If the book I want isn’t there, I go to half.com next. And then I go to Amazon to compare the price of books and shipping.
Anyway, I know that’s not what you were asking for, but I wanted to share.
Hope it was helpful at least! 
Lindsey