Anker is a solid brand in batteries, and 20000mAh is quite a bit of power. Put the two together and you have a compelling product. Drop the price down to $26.99, and you have a good deal.
On Amazon, you can currently find a good deal. The Anker PowerCore 20100 battery pack has a 20000mAh capacity and two USB ports. The box includes a single microUSB cable and a travel pouch.
Now here's what you don't get. QuickCharge. USB Type C. If either of those are deal breakers, look elsewhere. This pack is not going to charge your Nexus 6P in 30 minutes. Instead, you're looking at charging some other microUSB device in two or three hours, and the battery itself may take over ten hours to recharge.
But if you're like me, most of the gadgets in your home still have microUSB ports. And besides, USB 3.0 to Type C cables aren't exactly hard to find. This is a solid Prime Exclusive deal, and it ends in just under nine hours.