They will not give me a prepaid label to return a book I didn't order, can't afford and do not want.

This has happened previously and I can't afford to pay for returning postage for a book I never ordered!  Should I just keep the book and give it to someone and just not pay?  I didn't think you had to pay for something you didn't order.  They say it is easy to return and not owe a cent!  WELL HOW DO I DO THAT?