I know about Alibris ($49+ and it has to be “Alibris” books, not the others they have listed) and Seashell Books (much smaller stock, minimum $29+). Are there any others?
EdwardRHamilton.com isn’t free, but they’re $3.50 for any number of books, which is an even better deal.
Powells.com has new and used books and free shipping on orders over $50 (some items, shipped from another warehouse, are not eligible).
BetterWorld.com has free shipping in the US and only $3.95 for worldwide shipping.
Also check out bookfinder.com to search for books available thru thousands of booksellers.
I always recommend bookfinder.com for used books, but they all have shipping charges (subject to any special deals you can find on the indivdual sites it searches).
Abebooks has items with free shipping - enough that you can limit your results - once you search, there are drop down menus that let you choose this - but it is not generally free across the board (and shipping costs are set by the seller).
If you don’t mind putting up some of your duplicate or old books, you can get free shipping at Paperback Swap.com. And if you don’t have any books to off-load, you can buy credits for $3.45 a piece (one credit = one book).
for any and all book-searches, I always use Addall - it compares prices, shipping rates and estimated shipping time. I’m not sure if the vendors addall looks at offer free shipping, but you’'ll find out soon enough.