Design for: Xiaomi Pen, Product Name: Ink Refill, Pen Lead Diameter: 0.5mm.
Just check coupons out (with price alert and history). What is the 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 coupon code price check (UTC time):
Banggood
Banggood warehouse is located in China. Coupons for 15 Pcs Xiaomi Mijia Pen 0.5mm Ink Pen Refill? Nope, there’re no coupons in Banggood.
Link: Banggood product page
Price: $-3
It looks like this item is out of stock in Banggood. If you found it then please send me the link. I will replace it.
Click here to search on Banggood!
Additional Banggood coupons: first page and second page
Old coupons (from the oldest to the newest): MLREFILL15, 502C6B, TTxBG10, aff7off, bgssaff9, AFfbG10, BGVDay10%off, BGAFF10OFF, bGhOnEy10, BGdragonBoat, bgshOOp10, BGMP796, BgoFErTaSchInA10, BgcLuBofErtA10, BGMPM796, 13f142, Terence
(I don’t think these’re gonna work, but you can try ’em out anyway)
Other warehouses/versions
This item is available in 1 another warehouse/version!
1. Europe – 15 Pcs Xiaomi Mijia Pen 0.5mm Ink Pen Refill [EU] ($-3)
Price history and price alert
Here is price history (for all/most countries) for 15 Pcs Xiaomi Mijia Pen 0.5mm Ink Pen Refill. The lowest price was: $8.49.
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)).