Groupon offers the uNu Power DX 1,700mAh Rechargeable Battery Case for iPhone 4 and 4S in Glossy White or Matte Black for $29.99 with free shipping. That's the lowest total price we could find by $5, although we saw it for a buck less in February. It includes a transparent screen protector, cleaning cloth, and charging cable.

Also on offer is the uNu Power DX Plus 2,400mAh Rechargeable Battery Case for iPhone 4 and 4S in Black, White/Silver, or Black/Silver for $39.99 with free shipping. That's $3 under our January mention and the lowest total price we could find by $8. It purports to double an iPhone's battery life. Deals end September 7.