This deal has ended.
Get a Mophie Juice Pack Boost 2000mAh Portable Battery Pack for iPhone (30-pin) for $19.99 plus free shipping at Groupon Goods. This is the lowest price we found by $29, most places (Amazon was selling it for only $9 more, but there were only 9 were in stock and shipping adds $5). This battery can charge all models of iPhone up to the 4/4s models, as well we 30-pin iPods. It can also be configured into an iPhone stand that holds your device in portrait mode. It's good for up to 500 cycles. Sales tax is charged in some states.