eTrond via Amazon offers the Trond Air 4,000mAh Ultra-Slim External Battery Pack in Black (pictured), Blue, or Hot Pink for $19.99. Coupon code "P5GGIKVX" cuts it to $14.99. Plus, Prime members bag free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, it adds $6.29.) That's $25 off list and the lowest total price we could find. It features a flat micro USB cable, 1.5A input port, and PU leather case.