By Cam Vigliotta REI Co-op’s latest sale on outdoor gear and apparel is one for the ages. Right now, Co-op members can take 20% off one full-price item and an extra 20% off one discounted item from the REI outlet. The sale takes place March 18 to 28, and members need only enter the code MEMBER20 at checkout to receive the rare discount. For those unfamiliar with REI’s membership program, it goes a little something like this: by paying a one-time fee of $30 to the Co-op, you’ll receive a handful of lifetime benefits that include free standard shipping, 20% off shop services, 10% back on eligibl…