MegaMacs offers the MiLi Power Pack for Apple iPhone and iPhone 3G / 3GS in Black for $39.99. (The iPhone 4 and 4S are not supported.) With $5.21 for shipping, that’s the lowest total price we could find by $10. The MiLi Power Pack is an external 2000mAh battery that snaps onto the iPhone. It provides up to an additional 6.5 hours of talk time, 9 hours of video, or 31 hours of audio. It also charges via USB.