Aliexpress
Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China. I have no active coupons for you right now.
Link: Banggood product page
Price: -3 USD
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
Old coupons (from the oldest to the newest): BGDigoo15%, BGZXG30, BGSAMH, 20%DG422, BGDGZXG30, BGSMOFF, BGDGZX30A
(I don’t think these’re gonna work, but you can try ’em out anyway)
Price history and price alert
Price history (for all/most countries) for DIGOO DG-ZXG30 433MHz 2G GSM WIFI Smart Home Security Alarm System is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).