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I would also make the suggestion to visit your local libraries FOL sale and get some more books to post. Also check with friends & family to see if they have some they would let go of for you.
If you live in an area with a Freecycle email list, try posting to ask people if they would sort their bookshelves to find books they no longer need or want. Be sure to state that you are not asking for these to resell, but to read and/or trade for other books that you will read. I've had good success with both these suggestions. Just be sure to thoroughly go over the books to make sure they are in decent shape and meet PBS rules.
Short of that, Steve's suggestion is a good one, you can't go to a UBS and get many good books for the price of a credit here!
I'm selling credits for $3 each (they're $3.45 in the kiosk, I think) to help w/ my postage costs. I have over 50 books to take to the P.O. today & that's really gonna hurt the checkbook! I can sell you as many as you want :)