Unifun via Amazon offers Prime members the Unifun 10,400mAh Waterproof USB Power Bank
for $13.99 with free shipping
. (Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping costs $6.74.) That's the lowest total price we could find by $4. It features a waterproof rating of IP66, dual USB outputs, LED flashlight, and LED power status indicator.
: As a reader found, coupon code "9TCN9DCA" cuts it to $9.99