You can create price alert here, copy coupons and view price 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 price update (UTC time):
ZigBee Wireless Connection / APP Control / Multi-purpose Smart Home Security Equipment
Banggood
Banggood warehouse is located in China. Hmmm… There is no active coupon in Banggood at this moment.
Link: Banggood product page
Price: $-3
It looks like this item is out of stock in Banggood. If you found it then please send me the link. I will replace it.
Click here to search on Banggood!
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): 10XMA, 20XMAQ, d173d2, 256310, DAQIR, 360878, BGMAQDS, affiliate6, aff7off, BG7AQDWS, bgssaff9, BGaDmITAd, BGAQDSS9, 13f142, BGhumankit, BGBFAFF2019, BGAQDSS1, BGCITYADS2019, BGZIGBEEPL, BGAQDS, BGAQ20, BGHOMEAPP, BGAQA15, BGAQDS04, bGhOnEy10, BGSM15, BGSMOFF, BGAQWDS7, BGAQADOORS, bgshOOp10, BGAQDSUK, BGSAMRTH, BGN7CND, BgcLuBofErtA10, BGAQDSH, BGAQDSB, BGAQDSU, BG15SH12, BGAFF10OFF, BGSTHO15, BGAQZUK, BGAARDS1, BGCuponation10, BGAARDS2, BGAQDSSS, BG3d0660, BGd2c614, BGHONEY2110, BGSHOM15, BG571878, BG133c4e, BG40bece, BG200eff, BG78a9e0, BG87e1d2, BG46ecc4, BG0086c3, BGZJ323, BGGROC323, BGcou323, BGSLO323, BG115048, BG35eaee, BG015156, BGCL1323, BGCL323, BGHO323, BGcou925, BGNDH59, BG2f4c49, 6GSGIT, BGf3a810, BGD11047, BG15d2dc, BGB5059, ADMITAD2212, BGSmart6, BGHU1863, BGPLWIOSNA62, MVC8SPRING, 8BGwbg2204, pROMo8cuPoN, BGJUNEGSG9, Cuponation9, 9OFFSUMMER, BG8ReviTrage
(there’s a little possibility that old coupon will work)
Other warehouses/versions
This item is available in 1 another warehouse/version!
1. Europe – Xiaomi Aqara Window Door Sensor [EU] ($-3)
Price history and price alert
Here is price history (for all/most countries) for Xiaomi Aqara Window Door Sensor.
Price records source(s): Gearbest, 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)).