Booktopia is an online bookstore based in Australia, that offers:
Free shipping isn't available to everyone. Booktopia sends you an email when that option is available. During that time, you can pass the code to a third party (friend, family member, colleague, etc.). This is done because their shipping price is fixed, no matter how many things you order in one go. It's always $6.95.
This store offers their services only in Australia (including the rural areas) and New Zealand.
Paperback Swap is an online bookstore that is a little different because it lets you swap books with other users on the site. If you have a book to offer, you list it on your profile. For each book you send, you get one credit. You can use that credit to request a book from someone else, or to get a discount for the books this store offers.
For registering on this site, you get two credits, which means two books free of charge.