This Keystone ECO Booster Portable Battery Pack For Android Smart Phones drops from $39.99 to $9.99 at Tiger Direct. Shipping starts at around $4. (Tiger Direct also offers this battery pack for the same price through Amazon, but the shipping is locked at $7.28). This is the best deal we could find for one of these 1400 mAh batteries by $10. It has a retractable micro USB cable, which is only compatible with Android smartphones. There is no sales tax (except FL, IL, NC, TX, GA, SD and ID).