1byone via Amazon offers its 1byone 3,100mAh Battery Case for iPhone 6 in four colors (Black pictured) for $36.99. Coupon code "PBFVVP8I" cuts it to $29.99. With free shipping, that's $50 off list and the lowest total price we could find. It's Apple MFi certified and purports to add up to 14 hours of extra use time.