Amazon offers Prime members the Coolwoo Kids' Backpack in Dog for $16.99. Coupon code "4MQXAA86" cuts it to $11.89. With free shipping, that's the lowest the lowest total price we could find by $9. (Non-members can spend $49 or more to qualify for free shipping; otherwise, shipping costs $6.14.) It's water-resistant and BPA-free and features padded, adjustable shoulder straps.