Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China. There is one great coupon!
Link: Banggood product page
Coupon (24.99 USD): BGbeauty001
(CouponsFromChina.com tip: this code has to be applied 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): vipupgrade8, BGMi2i, BGoo1, BGHHH2, BGOMG23, BGHB20
(I don’t think these’re gonna work, but you can try ’em out anyway)
Price history and price alert
Price history (for all/most countries) for Xiaomi Maoxin 52C Hand Warmer 5000mAh is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).