Altatac via Rakuten offers the High Sierra Wave 70 Lightweight 2-Liter Hydration Backpack in five colors for $19.99 with free shipping. Apply coupon code "LIGHTNING15" to bag a $3 Rakuten credit. That's tied with our mention from two weeks ago (which didn't include the credit) and the lowest total price we've seen. (It's a current low by $6.) It features a 2-liter bladder, mesh straps, and zippered front accessory compartment.