Click through this link to access the offer page and the search box on AbeBooks — there are a lot of books that ship free (be sure to note the book's condition, though). It's hard to browse all free shipping books — you'll need to use the search box and enter some kind of search term, so if you don't really know what you're looking for, just do a search for "the" or "my" in the title field and it'll pull up a ton of books. You can then refine your results by signed books if you want.
Not sure how long this will last — it says "for the holidays" so it probably ends soon.
Forward your confirmation email to support to get 3% cash back, too!