Pen Lead Diameter: 0.5mm, Product Weight: 20g, Design for: Xiaomi Pen, Product Size (Lx Wx H): 13x 0.5x 0.5 cm.
Just check coupons out (with price alert and history). Best price?
Aliexpress
There’re some similar (or not) items on Aliexpress
You can use link bot to get even lower price:
Click here to search by yourself. Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China. Well, there is one coupon available right now.
Link: Banggood product page
1. ($8.99): BGAFF10OFF
just apply this code in your cart after clicking “Proceed to checkout”. Click to copy code and open shop.
Additional Banggood coupons: first page and second page
Old coupons (from the oldest to the newest): MLPEN799, e2bcea, TTxBG10, ea5394, bgssaff9, AFfbG10, BGTT10, bGhOnEy10, BGdragonBoat, BGMS042, BGWE833, BgoFErTaSchInA10, BgcLuBofErtA10, 00c906
(system checks these coupons on every price check (like once per 24 hours), but you can check them out anyway)
Other warehouses/versions
This item is available in 1 another warehouse/version!
1. Europe – Xiaomi Mijia Smooth 0.5mm Pen WIth 3Pcs Black Ink Refill [EU] ($-3)
Price history and price alert
Do you need price history chart (for all/most countries) for Xiaomi Mijia Smooth 0.5mm Pen WIth 3Pcs Black Ink Refill? There we go. The lowest price was: $6.99.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood and Aliexpress (only if product name will be the same as on Banggood and I will post it on social media though, so it’s more like Banggood price alert)).