Unifun via Amazon offers the Unifun 10,400mAh Waterproof Portable Battery Pack, model no. U821, for $16.99. Coupon code "ZUIN3F8E" drops it to $13.99. Plus, Prime members bag free shipping. (Non-members can spend $49 to bag free shipping; otherwise, it adds $6.74.) That's tied with our October mention and the lowest total price we could find by $6. It features a waterproof rating of IP66, dual USB outputs, LED flashlight, and LED power status indicator. Deal ends December 30.