Amazon offers Prime members the KidKraft Robot Medium Backpack, model no. 40106, for $14.36 with free shipping. (Non-members can spend $49 or more to qualify for free shipping; otherwise, it costs $6.29.) That's the lowest total price we could find by $21. It measures about 11" x 6" x 15" and features a large main carrying compartment with tablet sleeve inside, front zipper pocket for smaller items, and an adjustable padded shoulder straps for comfort.

Update: The price has dropped to $11.41.