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Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China. I can’t find any active coupons for this product in Banggood.
Link: Banggood product page
Price: -3 USD
It looks like this item is out of stock in Banggood. If you found it then please send me the link. I will replace it.
Click here to search on Banggood!
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k
Old coupons (from the oldest to the newest): 4BLF, BGHONEY2110, BGNPB4, FIN1860, BG10OFFAFF, BGwhielabel, BG10OFFAFF1, BANGGOODSD, 6GSGIT, BGTorch4, 00c906
(there’s a little possibility that old coupon will work)
Price history and price alert
Check out price history chart (for all/most countries) for Nitecore NPB4 20000mAh Power Bank.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).