Basically copy coupon, view price history or create e-mail price alert. Let’s see what best price is ATM.
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 code price check (UTC time):
Working humidity 0-95% RH / no condensation, Model MCCGQ14LM, Working temperature -10°C – 50°C, Sensing distance Max . 22mm, Size 41x 22x 11 mm, Name Aqara Window and Door Sensor E1, Wirelesss connection ZΙgbee 3.0
Banggood
Banggood warehouse is located in China. There are no active coupons for this product, but the price is pretty low isn’t it?
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): BGE1F982, 6GSGIT, BGa73610, BGa9dd07, BG5e1ded, BGD11045, BGD11AQA, BGB5057, BG05c26c, BG41b105, BG67240e, BG4aacf5, BG10afb9, BGd25e66, BGSmart6, BG1a8051
(sometimes old coupons work, so you can check them out (this site checks them on every price update))
Price history and price alert
Check out price history chart (for all/most countries) for 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)).