Aliexpress
Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China. I don’t think there is an active coupon right now – at least in my database.
Link: Banggood product page
Price: -3 USD
It looks like this item is out of stock in Banggood. If you found it then please send me the link. I will replace it.
Click here to search on Banggood!
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k
Old coupons (from the oldest to the newest): BGJUNEGSG9, BG15Sonoff, TTSUMMER8, Cuponation9, 9OFFSUMMER
(you can check these old coupons out, but these probably won’t work)
Price history and price alert
Check out price history chart (for all/most countries) for SONOFF 100-240V 50/60Hz 16A S-MATE Smart Switch Module eWeLink.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).