Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
Coupon (2.96 USD): BG10OFFAFF
Coupon (2.96 USD): BGwhielabel
(CouponsFromChina.com tip: this code has to be applied in your cart 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): BG2386, BGMASK004, vipupgrade8, BGUS20s, BGHPB7, bgshOOp10, bGhOnEy10, BGSD25698, BgcLuBofErtA10
(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 20Pcs Disposable Mouth Face Masks.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).