Aliexpress
Click here… to search for Xiaomi Cold Cathode UV Sterilization Pen on Aliexpress
Here are coupons that you can use.
Latest coupon code price check (UTC time):
Banggood
Banggood warehouse is located in China. There is one coupon right now! Check this out.
Link: Banggood product page
Coupon (26.66 USD): BGHealth06
(CouponsFromChina.com tip: paste coupon code in your cart and click Apply 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): ML15%Pro, BGPEN2, BG2TED, vipupgrade8, BGMIB2, 502C6B, 0e7f33, FBCOUPON1, BGwhielabel, 13f142, BG10OFFAFF1, BANGGOODSD
(these codes were not working in latest coupon update, but you can check them out anyway)
Price history and price alert
Here is price history (for all/most countries) for Xiaomi Cold Cathode UV Sterilization Pen.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).