Just check coupons out (with price alert and history). Best price?
Dimensions: 6.31x 3.22x 0.83 in (160.4x 81.8x 21 mm), Cell Capacity: 20000mAh at 3.6V or 72Wh, USB-C: 5-20V MAX 45W (15/3A) or 40W (20V/2A);, Battery Type: Lithium-ion cell, Color: Blue, Output: The combined power output over three ports is 45W, USB-A: (x2) 5V/2.4A 9V/2A 12V/1.5A MAX 18W (two ports total)., Operating Temperature: 32° to 113°F(0° to 45° C), Weight: 14.29 oz (405 g), MAX., Input USB-C: 5-20V MAX 45W
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):
Banggood
Banggood warehouse is located in China. I’m afraid that there’s no working coupon in Banggood.
Link: Banggood product page
Price: $-3
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 and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): BGCITYADS2019, aff7off, bGpAnDacHeCk, 2020CNY, BGXMI15, BGdragonBoat, BGAFF10OFF, bgshOOp10, BgoFErTaSchInA10, Email5, BgcLuBofErtA10, BG15POWERBANK, BGCuponation10, 13f142, BGHONEY2110, 7bcdf9
(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 Xiaomi ZMI QB820 PD 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)).