Sunvalleytek via Amazon offers the RAVPower 8,400mAh External Backup Battery Pack in Black, model no. RP-PB12, for $29.99. Coupon code "ZXZHF3OO" cuts it to $19.99. Plus, Prime members bag free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise shipping costs $5.55.) That's the lowest total price we could find by $10. It features dual 1-amp and 2-amp USB outputs.