Man how did I miss this for so long? Oh well, got it to work. Had a bit of trouble, it kept telling me that I couldn't use the coupon (the reason it gave seemed to say I had already used it?), but if anyone has any trouble try any/all of these tactics:
1). Log in first
2). Clear all cached data (Chrome made it easy to be able to clear the last hour - firefox you might have to just delete the specific cookies for B&N)
3). Try adding 2 movies to your cart and then the coupon, then delete one of the movies, then checkout. After logging in/out about 5 times, I somehow had an extra movie in my cart and the promo finally applied, and then remained after removing the second movie... this may have done the trick.