For a limited time you can get this Xcentz Portable Phone Battery Charger in red for only $8.99 when you use the promo code 3FNTGVFC at checkout. This has good reviews!
This portable charger is about the size of a tube of lipstick and comes with a handy LED flashlight. It can add a 130% charge to iPhone 8 / 7 / 6 / 6s, an 80% charge to iPhone X / 8 Plus / 7 Plus / 6s and one full charge to most other smartphones.
Shipping is free with Prime or your $25 purchase.