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Amazon has ENEGON 2-Pack Portable Power Bank 10000mAh, with USB C & Micro-USB Input and Dual USB Output [amazon.com] for $ 11.39 after clipping 5% coupon on product page and using code 358PILZQ at checkout. Shipping is free with Prime or on orders $ 25+.

10000mAh capacity charges an iPhone 8 3 times, iPhone X 2.6 times, iPhone 7 Plus 2.3 times, or a Galaxy S8 2.2 times.