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Better world books is having their summer sale and are offering 15% off if you purchase 4 or more used books. They also have free shipping to Australia, although it can take a while to arrive.
Although they are not a charity, they do give a portion of each purchase to various literacy programs around the world.
it's not 15% off all books such as text books and stuff. Only applies to the books in the dog summer sale category. Might find some good reads in there though.