Aliexpress
Click here… to search for Xiaomi Deerma USB Oil Diffuser on 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 (15.97 USD): BG10OFFAFF
Coupon (15.97 USD): BGwhielabel
Coupon (16.74 USD): 13f142
(CouponsFromChina.com tip: 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
Old coupons (from the oldest to the newest): AP1499, L2353H, 502C6B, 963205, vipupgrade8, d8f2bb, e9449a, 9de68e, 919c60, TTxBG10, BG1155DM, bgssaff9, BG1099DM, AFfbG10, BGBINKUPON414, BGBFAFF2019, BGCITYADS2019, BGGRMHWPA, BG12dem, BGVDay10%off, DMRT52, BGHOMEG12, BGdxf52, BGAFF10OFF, bGhOnEy10, BGDem254, BGdragonBoat, BGSS10%SW, bgshOOp10, BGHome15%, BGHOMEA15%, BgcLuBofErtA10, BGDEFO414, BGCuponation10
(system checks these coupons on every price check (like once per 24 hours), but you can check them out anyway)
Price history and price alert
Here is price history (for all/most countries) for Xiaomi Deerma USB Oil Diffuser.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).