Terms and conditions at the bottom of this page:
http://www.rei.com/outlet/coupons/20pctcoupon.html
Free shipping if you pick up your order at a local REI store.
Should stack with FatWallet Cash Back.
The discount is automatically applied when you checkout. No need to enter a coupon code.
Remember that REI dividends come out later this month, so you may not want to spend too much now if you're expecting a big dividend.