Aliexpress
Click here… to search for HCHO/TVOC/PM2.5/PM10 Tester Formaldehyde Detector on Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
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 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): BG82e6ed, BGherra, BG9d5aac
(probably won’t work, but you can try them out)
Price history and price alert
Here is price history (for all/most countries) for HCHO/TVOC/PM2.5/PM10 Tester Formaldehyde Detector.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).