Fosto via Amazon offers its Fosto Ultra Slim 3,100mAh Battery Case for iPhone 7 in Black for $37.99. Coupon code "6CGW7DSV" cuts that to $22.79. With free shipping, that's the lowest price we could find for such a case by $5.
It's also available for iPhone 7 Plus for $41.99. Coupon code "ZHMS6V2E" cuts that to $25.19. With free shipping, that's the lowest price we could find by $4.