Amazon offers Prime members the Topeak Aero Wedge Pack Strap-On Seat Mounted Bag, model no. 63002260, for $5.24 with free shipping. (Non-Prime members can spend $35 or more to qualify for free shipping; otherwise, shipping adds $5.58.) That's a buck under our mention from last month and the lowest total price we've seen. (It's a current low by $8.) It fits 27.2mm to 34.9mm seatposts and features a quick-release mounting system, nylon construction, 3M reflective strip, and safety light clip.