Groupon offers the Patriot Fuel 9,000mAh Portable Dual USB Battery Charger for $49.99 with free shipping. (Amazon has it for the same price.) That's the lowest total price we could find by $20. It features one 2.5A and one 1A USB port for simultaneous charging and it's compatible with tablets, smartphones, and MP3 players. Offer ends September 12.