Amazon offers Prime members the Aduro PowerUp 11,000mAh Portable External Battery Pack
in six colors (Black pictured) for $18.99
with free shipping
. (Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping costs $6.55.) That's the lowest total price we could find by $4. It features two 1-amp ports and one 2.1-amp port to charge two phones and a tablet simultaneously. A travel bag and USB cable are included.