Just check coupons out (with price alert and history). What is best price?
Aliexpress
There’re some similar (or not) items on Aliexpress
Click here to search by yourself. Here are coupons that you can use.
Latest coupon code price check (UTC time):
Rated Energy 13 /500mAh 5V (TYP 2A), Input: USB-C (INPUT) 5V=2.4A / 9V=2A / 12V=1.5A (18W MAX) USB-C (IN): 5V=2.4A / 9V=3A / 12V=2.5A / 15V=2A / 20V=1.5A (30W MAX), IP Rating IPX5, Capacity 20 /000mAh 3.85V (77Wh), Weight 11.45oz, Capacity 20 /000mAh, Output: USB-C (OUT1) 5V=2.4A / 9V=3A / 12V=3A / 15V=3A / 20V=2.25A (45W MAX) USB-A (OUT2/OUT3): 5V=2.4A / 9V=2A / 12V=1.5A Multi Ports: 5V=3A (MAX), Maximum Brightness 1 lumen, Dimensions L-5.37″xW-2.36″xH-0.84″, Weight 324.5g (11.45 oz), Battery Type 21700 Li-ion Batteries
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
1. ($98.13): BGAlerabat15
2. ($103.90): BGtorch2
coupon code has to be applied in shop’s cart after clicking “Proceed to checkout”. Click to copy code and open shop.
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): 4BLF, BGHONEY2110, BGNB20000, FIN8358, BG10OFFAFF, BGwhielabel, BG10OFFAFF1, BANGGOODSD, BGTorch4, 00c906, BGtorch3
(probably won’t work, but you can try them out)
Price history and price alert
Check out price history chart (for all/most countries) for Nitecore NB20000 20000mAh Power Bank.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood and Aliexpress (only if product name will be the same as on Banggood and I will post it on social media though, so it’s more like Banggood price alert)).