Aliexpress
Click here… to search for Xiaomi 0.5mm Metal Sign Pen on Aliexpress
Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China. I have no active coupons for you right now.
Link: Banggood product page
Price: -3 USD
It looks like this item is out of stock in Banggood. If you found it then please send me the link. I will replace it.
Click here to search on Banggood!
Old coupons (from the oldest to the newest): 502C6B, e2bcea, TTxBG10, 2019SPEDM
(you can check these old coupons out, but these probably won’t work)
Price history and price alert
Here is price history (for all/most countries) for Xiaomi 0.5mm Metal Sign Pen.
Price records source(s): Gearbest, Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).