Xiaomi Smartmi 3Pcs PM2.5 Mask (Banggood, Aliexpress: filtration efficiency of more than 97% PM2.5, 3x XIAOMI Smartmi Masks (the best price with coupon)).
Price history, coupons and price alert are available on this page. You probably want to know what is the 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 coupon and price update (UTC):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
1. ($19.28): Cuponation9
2. ($19.28): 9OFFSUMMER
3. ($19.49): BG8ReviTrage
you should apply the coupon in your cart after clicking “Proceed to checkout”. Click to copy code and open shop.
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): HHHHHOT, healthylifestyle, BGSMDM0, vipupgrade8, BGHC20, 502C6B, BG4ee866
(you can check these old coupons out, but these probably won’t work)
Price history and price alert
Do you need price history chart (for all/most countries) for Xiaomi Smartmi 3Pcs PM2.5 Mask? There we go. The lowest price was: $7.99.
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)).