Sunvalleytek via Amazon offers the RavPower Element 10,400mAh Portable USB Charger in Black or White, model no. RP-PB07, for $24.99. Coupon code "P3FFG8XA" cuts it to $16.49. Plus, Prime members receive free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping costs $6.59.) That's $1 under our March mention and the lowest total price we've seen. (It's also a current low by $11.) It features 1-amp and 2.1-amp USB outputs.