Aliexpress
Click here… to search for Xiaomi DEERMA DEM-ST300S 4L Humidifier on Aliexpress
Here are coupons that you can use.
Latest coupon code price check (UTC time):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
Coupon (79.19 USD): BG10OFFAFF
Coupon (79.19 USD): BGwhielabel
Coupon (86.99 USD): 13f142
(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): 502C6B, PICODICOM, 919c60, BGDEER05, aff7off, bgssaff9, BGaDmITAd, bgchinafest, BGBFAFF2019, BGCITYADS2019, BGST300S, Email5, 2020CNY, BGHOMEAPP, bGhOnEy10, BGSS10%SW, bgshOOp10, BGHome15%, BGHOMEA13, 7d18f7
(there’s a little possibility that old coupon will work)
Price history and price alert
Here is price history (for all/most countries) for Xiaomi DEERMA DEM-ST300S 4L Humidifier.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).