Groupon offers the Aduro PowerUp 4,000mAh Backup Battery in Black, Purple, or Turquoise for $14.99 plus $3.99 for shipping. That's $35 off list and the lowest total price we could find, although we saw it for $2 less in July. It features two USB ports, a flashlight, microUSB charging cable, and carrying pouch. Deal ends September 22.
Also available are the Aduro PowerUp 5,200mAh Backup Battery for $19.99 plus $3.99 for shipping and the Aduro PowerUp 11,000mAh Backup Battery for $29.99 with free shipping, price drops of $40 and $50, respectively. (Each was 10% less back in July.)