TomTop offers the Free Knight 50-Liter Outdoor Sport Backpack in Green for $14.99 with free shipping. (Choose "United States Warehouse" under "Ships From" to see this price. It's available in several additional colors for a buck more.) That's a buck under our mention from a month ago and the lowest price we could find now by $4. This water-resistant pack features multiple exterior pockets, a padded back support, ergonomic padded shoulder straps, and adjustable chest and waist belts.
Note: The countdown timer on the product page does not reflect an actual expiration time and resets daily.