I currently have a 3-1 on all paperbacks and 2-1 on all hardcover going on my shelf.  I have had several people order a paperback and then request 2 larger, heavier hardcover books.  I was looking to do 3 paperbacks for 1 and 2 hardcover books for 1.  How can I change the wording so that it isn't confusing.

Theresa

Maybe something like this: two free paperbacks with any order (3:1) or order a hardcover and get any second book free (2:1). 
I think cost wise this makes sense. The ordered book is the one that earns a credit but due to weight postage will vary. Your second option of mailing 2 HC together is usually more expensive than mailing 2 or 3 PB together or even sending 1 HC plus 1 or 2 PB. Offering a second HC as a freebie is where your postage cost will run higher. HTH