Shoebuy offers the CalPak S Curve 18" Backpack
in six color choices (Navy Blue/Charcoal/Cool Gray/Black pictured) for $25.45. Coupon code "EMLSFLING12" cuts it to $15.45
. With free shipping
, that's the lowest total price we could find by $5, outside of the price below. It measures 18" x 13" x 7" and features one large main compartment and a zippered pocket on the front panel.
A close price: eBags via eBay offers it for $15.99 with free shipping.