Anker Direct via Amazon offers the Anker PowerCore 20,000mAh Portable USB Charger in Black or White, model no. AK-A1271011, for $39.99. Coupon code "HAVKXACP" cuts it to $29.99. Plus, Prime members bag free shipping. (Non-members can spend $49 or more to qualify for free shipping; otherwise, shipping costs $6.63.) That's $70 off and the lowest total price we could find. It features PowerIQ technology and is compatible with most USB powered products.