LightInTheBox offers the Cheng Xintu 60-Liter Water-Resistant Backpack in seven colors (Red pictured)for $37.99. Coupon code "5525025BACKPACK" drops it to $18.99 plus $8.64 for shipping. That's the lowest total price we could find by $3. It features an adjustable chest belt, adjustable waist belt, and detachable internal aluminum frame.

Note: This item ships from China and may take two to five weeks to arrive. Uncheck "Add Shipping Insurance to your order" during checkout to remove the 49-cent fee.

Update: It's now available with free shipping.