Aliexpress
Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China. I found coupon for you. Check this out.
Link: Banggood product page
Coupon (82.46 USD): BGSmart6
(CouponsFromChina.com tip: apply code in your basket (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): ADMITAD2212, BG15Sonoff
(you can check these old coupons out, but these probably won’t work)
Price history and price alert
Price history (for all/most countries) for 10pcs SONOFF ZBMINI Zigbee3.0 Two-Way Smart Switch is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).