Just check coupons out (with price alert and history). What is the best price?
Aliexpress
There’re some similar (or not) items on Aliexpress
Click here to search by yourself. Here are coupons that you can use.
Latest coupon code price check (UTC time):
Product size: 58.7 × 30 × 13.5mm
Banggood
Banggood warehouse is located in China. Only one coupon available!
Link: Banggood product page
1. ($6.11): 2203ActiesNL
paste coupon code in your cart and click Apply after clicking “Proceed to checkout”. Click to copy code and open shop.
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
There were country-specific coupon codes published on Couponsfromchina social channels and price alerts. Coupon price may vary by country, because there’re different VAT rates, shipping cost etc. There is a chance that some latest coupons are still working:
Old coupons (from the oldest to the newest): elec, e2bcea, domoticachollos, BG15Gadgets, SONOFF15, BGVDay10%off, bGhOnEy10, BGSMOFF, BGSONOFF15, BGSAMRTH, BGELIG0920, BgcLuBofErtA10, BGSONOFF16, BGAFF10OFF, BGSTHO15, BGHONEY2110, BGSHOM15, 6GSGIT, ADMITAD2212, BGSmart6, MVC8SPRING, BG15Sonoff
(sometimes old coupons work, so you can check them out (this site checks them on every price update))
Other warehouses/versions
This item is available in 1 another warehouse/version!
1. Europe – SONOFF 433MHz Remote Controller [EU] ($-3)
Price history and price alert
Price history (for all/most countries) for SONOFF 433MHz Remote Controller is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood and Aliexpress (only if product name will be the same as on Banggood and I will post it on social media though, so it’s more like Banggood price alert)).