Romoss Direct via Amazon offers its Romoss Ultra Slim 2,000mAh Extended Battery Case for iPhone 6/6s in White or Grey for $19.99. Coupon code "ROMOCASE" cuts that to $16.99. Plus, Prime members bag free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping costs around $6.) That's the lowest total price we could find by $6. It purports to provide up to 85% extra battery power and supports charge and sync with the original lightning cable.