Go down and check coupons out. Remember to check price history if it is available. What is 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):
0.300~1.000: Moderate, Item size: 164 * 74 * 26mm / 6.5 * 2.9 * 1.0in, 0.100~0.300: Slight, 0~0.100: Fine, O₃ concentration AQI: , Color: black+orange, 1.000~4.999: Serious, Material: ABS, Package size: 280 * 225 * 70mm / 11.0 * 8.9 * 2.8in, Package weight: 501g / 1.1lb, Battery: 1 * lithium battery / 2000mAh (included), Screen size: 2.8in
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): BGGines
(I don’t think these’re gonna work, but you can try ’em out anyway)
Price history and price alert
Check out price history chart (for all/most countries) for Ozone Analyzer Meter Sensor.
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)).