Aliexpress
Here are coupons that you can use.
Latest coupon code price check (UTC time):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
Coupon (6.27 USD): Cuponation9
Coupon (6.48 USD): BG00c906
(CouponsFromChina.com tip: you should apply the coupon in your cart after clicking “Proceed to checkout”. Click to copy code and open shop)
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k
Old coupons (from the oldest to the newest): BGd0b05d, BG2589f4, 00c906, ADMITAD2212, 2022ACTIESNL, Terence, Tabby20, 8Couponsde, MVC8SPRING, BGJUNEGSG9
(these coupons are old, so probably won’t work)
Price history and price alert
Check out price history chart (for all/most countries) for Dual USB 5V 1A 2.1A Power Bank 18650 PCB Module Board.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).