Just check coupons out (with price alert and history). Let’s see what best price is ATM.
Protocol Zi-Bee, Weight 150g, Model Window & Door Sensor (Zi-Bee Version), Size (Lx Wx H) 4.10x 2.20x 1.10 cm / 1.61x 0.87x 0.43 inches, Brand Aqara (Xiaomi Ecological Chain)
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 and price update (UTC):
Banggood
Banggood warehouse is located in China. I don’t think there is an active coupon right now – at least in my database.
Link: Banggood product page
Price: $-3
It looks like this item is out of stock in Banggood. If you found it then please send me the link. I will replace it.
Click here to search on Banggood!
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): BGc94a9b, BGAQA929, BGD11059, 8BGwbg2204, 2203ActiesNL, tRaDetRaCKeR, 8wEbgaInS5, pROMo8cuPoN, Cuponation9, BGXIFD371
(there’s a little possibility that old coupon will work)
Other warehouses/versions
This item is available in 1 another warehouse/version!
1. Europe – 2 pcs. Aqara E1 Window Door Sensor Zigbee 3.0 [EU] ($-3)
Price history and price alert
Check out price history chart (for all/most countries) for 2 pcs. Aqara E1 Window Door Sensor Zigbee 3.0.
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)).