Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
Coupon (27.99 USD): BGa66594
Coupon (38.21 USD): Cuponation9
Coupon (38.63 USD): BG8ReviTrage
(CouponsFromChina.com tip: put item in your cart and apply coupon code there 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): BG7ac30c, BGHB218, BG2d31bc, BG72fff5, tRaDetRaCKeR, BG3fc23d, BGHECARE1, BGMUML65, BG8516c5, BG724b00, BG4ee866, BGb808ae, BG48681b
(these codes were not working in latest coupon update, but you can check them out anyway)
Price history and price alert
Price history (for all/most countries) for Wireless Far Infrared Heated Massage Knee Pad is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).