Yugster offers the UNU Enerpak 2,800mAh Portable External Battery 2-Pack in nine colors (Black/Grey pictured) for $10.97. Coupon code "FreeYugBuck" cuts it to $9.97. With $4 for shipping, that's the lowest total price we could find by $4. It includes a micro USB cable.
The countdown timer on the product page does not reflect an actual expiration time and resets daily.