Amazon wants to make a buck or so on the transactions of their vendors. That's one reason why they won't let you free ship. When you buy something from a vendor on Amazon, look at the actual cost of them shipping it to you. Then you'll see that Amazon sets their shipping charges so they can make a little too.
Free shipping is not available to Marketplace buyers at this time, sellers can refund a partial amount to a buyer. Many sellers give a shipping discount if the buyer purchases more than one book at a time and the seller can ship them all at once.