AnkerDirect via Amazon offers the Anker Astro Slim2 4,500mAh External Battery Power Bank, model no. AK-79ANS4552-BA, for $27.99. Coupon code "SLIM2AST" cuts it to $19.99. With free shipping, that's $30 off list and the lowest total price we could find. It features dual USB ports for simultaneously charging two devices and a micro USB cable.