A4C offers the SaFPWR Milano Rechargeable Smart Battery Case for iPhone 4 and 4S in Silver for $9.95 with free shipping. That's $29 under the lowest total price we could find for this case new in any color and the best price we've seen for a new unit this year. (For further comparison, we saw a refurb for just $2 less two weeks ago.) It purports to double the battery life for an iPhone 4 or 4S via its 2200 mAh battery.