Aliexpress
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Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China. You can get coupon down below.
Link: Banggood product page
Coupon (4.84 USD): BGSMOFF
(CouponsFromChina.com tip: put item in your cart and apply coupon code there after clicking “Proceed to checkout”. Click to copy code and open shop)
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k
Old coupons (from the oldest to the newest): BGZXD21, BGaDmITAd, BGDGZXD21, BGDigoo15%, BGBFCR01, aff7off, bGpAnDacHeCk, 2020CNY, BGVDay10%off, BGZXD211, BGSAMH, 20%DG422, BGDGDSG, BGSM15
(sometimes old coupons work, so you can check them out (this site checks them on every price update))
Price history and price alert
Price history (for all/most countries) for DIGOO DG-ZXD21 WIFI Door Window Sensor is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).