Aliexpress
Click here… to search for SONOFF Wireless Remote Transmitter on 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 (3.82 USD): BG15Gadgets
Coupon (4.05 USD): elec
(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): domoticachollos, e2bcea, BG15Sonoff
(I don’t think these’re gonna work, but you can try ’em out anyway)
Price history and price alert
Do you need price history chart (for all/most countries) for SONOFF Wireless Remote Transmitter? There we go.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).