This may be common knowledge already, but I was unaware...
REWARDS20 = 20% off one item with new REI membership
As a non-member if you buy anything over $100, make sure to add the REI membership to your shopping cart, and use promo code REWARDS20. The 20% discount will cover the entire cost of the membership.
I actually got a new Osprey Kestrel AND REI membership for about $10 less than I was planning on paying for the backpack alone.