The nice thing about this discount, other than that it's going on until June 10th according to this promo sheet I picked up today, is that you don't have to buy the EVO from Sprint to get it.
I got my EVO two weeks ago from eBay and don't even have a Sprint account (my phone came with a month of free service), but I still bought seven EVO accessories (five yesterday, two today) for 20% off. Store employees will try to get your name so they can check your account for some reason, but they leave you alone after you say you got the EVO off contract.
Sprint generally has the best selection of EVO accessories, so choosing two to get the discount isn't hard to do. Happy shopping!