Aliexpress
Click here… to search for Xiaomi 710ml U-Shape Hot Water Bag on Aliexpress
Here are coupons that you can use.
Latest coupon code price check (UTC time):
Banggood
Banggood warehouse is located in China. Only one coupon available!
Link: Banggood product page
Coupon (18.39 USD): BGeb5395
(CouponsFromChina.com tip: just apply this code 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): 99a5c0, 99a5co, BGOD1, AFfbG10, 0e7f33, BGTS204, FBCOUPON1, BGTRIP01, BGHOTBAG, BG5JT9, BGC202G, BGTY742TRL, BGCuponation10, BGHONEY2110, BG10OFFAFF, BGwhielabel, BG10OFFAFF1, BG95811f, BGAlerabatod, BGe46870, Terence, 2203ActiesNL, BG1efeb0, MVC8SPRING, 8BGwbg2204, tRaDetRaCKeR, 8wEbgaInS5, pROMo8cuPoN
(sometimes old coupons work, so you can check them out (this site checks them on every price update))
Price history and price alert
Price history (for all/most countries) for Xiaomi 710ml U-Shape Hot Water Bag is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).