You may add one free item for every five items purchased to your order. 

For example if you buy 1 - 5 items, you may add 1 free item.  If you buy between 6 and 10 items you may add 2 free items.  If you buy between 11 - 15 items, you may add 3 free items.  And so forth.

You may ONLY add ebooks as a free item if you've purchased ebooks.