I have now successfully used my in-store 20% off 3 times. I don't think they scan anything, the cashier simply just applies a 20% line discount to the most expensive eligible item. This makes me pretty happy and also sad for the company, but oh well. And no, I haven't been trying to con the cashiers, but both the second and third time I've bought something from the store last week they didn't even ask me about it, just rang it through. I even got an additional 5% off my new watch cause I'm a nice customer (I think the girl was having a rough day). So having good luck lately. If you don't have any moral dilemmas to deal with, just play dumb and you might get another free discount.
Assuming the online coupon will only work once for REI.com and REI-Outlet.