AnkerDirect via Amazon offers the Anker Zolo Power Protection Combo for iPhone 5/5S, model no. AK-C1041012, for $19.99. Coupon code "ANK0599S" cuts it to $5.99. Plus, Prime members bag free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping costs $6.37.) That's $24 off list and the lowest total price we could find. This combo pack includes a protective case and magnetic battery that attaches to the back of the case and purports to provide approximately one full additional charge.

Of note, Amazon also offers it for Galaxy 5S for the same price via the same coupon code. That's $14 off and another low.