You can get Xiaomi Mijia Smart Door Window Sensor 2 for $18.39 with coupon (from Banggood). Basically copy coupon, view price history or create e-mail price alert. Best price?
Latest coupon and price update (UTC):
Battery: CR2032(Built-in), Name: Door & Window Sensor 2, Working Humidity: 0%-95%RH, Wireless Protocol: bluetooth 5.1 BLE, Working Temperature: -10℃-50℃, Size: 34*32.7*14.5mm
Banggood ($18.39)
Banggood warehouse is located in Hong Kong. There is no active coupon in my database right now (for Banggood). I think you want to know if the price is good. Well, this price is higher than average price ($12.79)
Link: Banggood product page
Price: $18.39
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): 13f142, BGXMNEWD, BGSAMRTH, BGLTPX5ND, BgcLuBofErtA10, BGXMDS2, BG15SH12, BG2020FRM, BGSTHO15, BGXM2002, BGXMDSO, BGHONEY2110, BGSHOM15, 6GSGIT, BGf38540, BGD11038, BG616162, BGB5046, BG3c2af1
(these coupons are old, so probably won’t work)
Aliexpress
There’re some similar (or not) items on Aliexpress
![Xiaomi Mijia Smart Pump 2 MJCQB06QW [EU]](https://couponsfromchina.com/wpthumbnails/Resized1005006240191932.jpg)
![Xiaomi Mijia Smart Pump 2 MJCQB06QW [EU]](https://couponsfromchina.com/wpthumbnails/Resized1005006151871019.jpg)
Click here to search by yourself. Here are coupons that you can use.
Price history and price alert
Here is price history (for all/most countries) for Xiaomi Mijia Smart Door Window Sensor 2.
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)).