I have someone who wants about 50 books that I am about ready to donate. Would it be against site rules if instead of asking her for credits I just ask her to refund the amount of the postage??? 

Not sure, but it sounds like that might be edging into "selling books" territory, which goes against the one book = one credit rule. You could probably just arrange to do it off-site. But this sounds like a perfect reason to contact the admins and see what's kosher.

They have allowed people to ask for prepaid envelopes before. The reasoning there was that is was not currency though, the sender wasn't asking for something they could go use for anything other than sending the books to the requester. I don't think it would OK to outright ask for money.You could weigh the books and whatever you are going to send them in, and ask them to send you a prepaid label, or calculate how many credits you would need to sell to cover the postage and ask for that many credits. But I think if you said that they needed to send you $X, it really doesn't matter that it is based on postage costs, you would still be selling them the books for $X and we can't sell books.

I was going to suggest also that you weigh the books and box, and then just do a x/1 to cover the postage.  Someone sent me about 18 books in a computer paper box a few weeks ago and I think it was about $7.00 or so.  It was a mixture of paperback and hardback, so the postage wouldn't be that much all things considered.  Good luck.  Pat