Ask the sender to read the Help Center item "A requestor says there is a problem with a book I sent!"

Right in there, in big letters, it says "In all cases of Problem transactions, the receiver does NOT have to send the book back to the sender."

If the sender wants their book back, they either need to send you the money to cover the postage back to them or they need to send you an extra credit to cover the cost of the postage.  You do NOT have to send the book back at your expense.

You are NOT required to return the book to the sender.  If she wants it back, and you want to go to the trouble, she can send you TWO credits (one to cover postage), and you can send it back.  But I sure wouldn't return it without getting the credits first.

it is very possible with the cheaper paper that wrinkly pages do not always = water damage.  I think someone once said that the humidity in Florida creates some wrinkly pages. 

I'm not saying your book wasn't water damaged but just another possibility.

DH and I went to Cancun last year. Every book I brought with me wrinkled up, just from being in the humid air. But when I returned home, they straightened out again in our dry air. But I would think that some wrinkling would become permanent in a moist environment. But the wrinkles in the pages were definitely not water damage (as in spilled something on the pages). They were distinctly different. (Not sure I can describe it.)

This year, I plan to take pictures of a book I'm bringing with me before and after I get home as a little experiment. What I saw last year piqued my curiosity, LOL!