Shipping to the US via Media Mail is $6 for the first book and $1 for each additional. 

To Canadian addresses, shipping is $13 for the first book and $3 for each additional. 

The rest of the world is $14 per book via Global Priority.  With multiple purchases (4 books for sure, possibly 6 depending on their sizes)I may be able to use a Priority Box to save some money for you.