Aliexpress
Here are coupons that you can use.
Other warehouses/versions
This item is available in 4 another warehouse/versions!
1. China – SONOFF S20 Switch Socket EU Type F (-3 USD)
2. China – SONOFF S20 Switch Socket EU Type E (-3 USD)
3. China – SONOFF S20 Switch Socket UK Plug (-3 USD)
4. China – SONOFF S20 Switch Socket AU Plug (-3 USD)
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China. There are no coupons for this product in Banggood.
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): 502C6B, a3af93, e2bcea, elec, SONOFF15, BGBFCR01, 13f142, BG9NY, BGVDay10%off, BGTT10, bGhOnEy10, BGSMOFF, BG15Sonoff
(sometimes old coupons work, so you can check them out (this site checks them on every price update))
Other warehouses/versions
This item is available in 4 another warehouse/versions!
1. China – SONOFF S20 Switch Socket EU Type F (-3 USD)
2. China – SONOFF S20 Switch Socket EU Type E (-3 USD)
3. China – SONOFF S20 Switch Socket UK Plug (-3 USD)
4. China – SONOFF S20 Switch Socket AU Plug (-3 USD)
Price history and price alert
Here is price history (for all/most countries) for SONOFF S20 Switch Socket US Plug.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).