According to an email:
Everything on site (with a few exceptions) is 20 % off.
Also, most importantly, Free Shipping. But you must spend over $79 AFTER all the discount have been taken.
End 15th at 11:59 EST
So, if I have this right, you would need to buy 3 EarthBoxes at $32.95 each to get 20% off and the free shipping. (That would be 8 cents over $79.)
Of course you can mix and match whatever you want.
Just be mindful of the restrictions.