Aliexpress
Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China. Only one coupon available!
Link: Banggood product page
Coupon (159.79 USD): BG589b2e
(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): BGSaleFeb8, BGSaleAD, BGc28268, BGMARCHSALE
(there’s a little possibility that old coupon will work)
Price history and price alert
Price history (for all/most countries) for Solar Inverter Kit With 18W Solar Panel 30A Solar Controller is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).