NBOX via Amazon offers the kdLinks Supreme Power Case and Extended Rechargeable Battery for iPhone 4 and 4S for $37.99. Coupon code "E8NZ8HER" cuts it to $27.99. With free shipping, that's $2 below the lowest total price we could find for a similar (albeit heavier) iPhone power case elsewhere. It weighs 3 oz. and purports to double the battery life for an iPhone 4 or 4S.