BuyDig offers the uNu Exera Modular Detachable Battery Case for iPhone 4S in White/White (model UNU-EX-04-1700W-WHT) for $54.95 with free shipping.

What makes this deal hot:
You save a total of $15 off of the retail list price. That's the lowest price we could find currently. No tax in most states.

What this does & Why you need it:
This features 1700mAh Swappable Battery Module, adds additional 7 hours of talk time, Fully protected and more.