Size: 86*86mm Channel: 1 CH Remote Control Distance: 0-100m Encode: EV1527 Frequency: 433.92 Current: 20-25ma Power Supply: 12V 27A Battery (NOT included).
You can create price alert here, copy coupons and view price history. Let’s see what best price is ATM.
Aliexpress
There’re some similar (or not) items on Aliexpress
You can use link bot to get even lower price:
Click here to search by yourself. Here are coupons that you can use.
Latest coupon code price check (UTC time):
Banggood
Banggood warehouse is located in China. I am tired of searching coupons, but I found one for you…
Link: Banggood product page
1. ($10.75): elec
apply it in your cart after clicking “Proceed to checkout”. Click to copy code and open shop.
Additional Banggood coupons: first page and second page
Old coupons (from the oldest to the newest): 502C6B, e2bcea
(there’s a little possibility that old coupon will work)
Price history and price alert
Price history (for all/most countries) for 3x SONOFF Wireless Remote Transmitter is here. The lowest price was: $9.71.
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)).