Aliexpress
Here are coupons that you can use.
Latest coupon code price check (UTC time):
Banggood
Banggood warehouse is located in China. We have found one coupon for this product!
Link: Banggood product page
Coupon (32.56 USD): BG589b2e
(CouponsFromChina.com tip: apply it 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): BG6d2f05, hoNEYbGdeAlS, Cuponation9, BG8ReviTrage, BGcoupon8off, TTBLACK5
(system checks these coupons on every price check (like once per 24 hours), but you can check them out anyway)
Price history and price alert
Price history (for all/most countries) for AC 220V 4000W Variable Voltage Regulator is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).