Material: PC V0;, Wi-Fi: IEEE 802.11 b 2.4GHZ;, Quiescent current: ≤ 40 μA;, Not included);, Working Voltage: DC3V (2x 1.5V battery, Battery Model: AAA 1.5V;.
Just check coupons out (with price alert and history). You probably want to know what is the best price?
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 price update (UTC time):
Banggood
Banggood warehouse is located in Europe 
. I couldn’t find any coupon for this product on latest product update. Sorry, but there’s no price history in my database.
Link: Banggood product page
Price: $-3
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 and https://couponsfromchina.com/short/i1u
Old coupons (from the oldest to the newest): BGSONOFFDW2, BGSONOFF15, BGELIG0920, BgcLuBofErtA10, BGDW2, BGSONOFF16, BG1674, BGSTHO15, BGHONEY2110, BGSHOM15, BGSHE15, 13f142, 6GSGIT, ADMITAD2212, BGSmart6, MVC8SPRING, pROMo8cuPoN, BGJUNEGSG9, BG15Sonoff, BG8b589f, Cuponation9
(probably won’t work, but you can try them out)
Other warehouses/versions
This item is available in 1 another warehouse/version!
1. China – SONOFF DW2 Door/Window Sensor ($9.59)
Price history and price alert
Not enough data to show price history chart… $9.99 is the only recorded price. It was not cheaper than that since we have saved it in DB.
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)).