iWALK 3350mAh Ultra-Compact Power Bank

This 3350mAh is big enough to add almost 1.2 charges to an iPhone 8, or about 0.95 charges to an iPhone X. Just plug into your iPhone & charge, never bother with the charging cable again. Price drops from $25.99 to just $23.39 when you clip the 10% coupon on the product page. Shipping is free with Amazon Prime. Shipping is also free when you spend $25 or more. 

Clip the 10% coupon.