When I was updating this page, then coupon price was 14.99 USD for 3Pcs Xiaomi Mijia T100 Toothbrush Replacement Head (in Banggood). You can create price alert here, copy coupons and view price history. Best price?
Latest coupon code price check (UTC time):
Banggood (14.99 USD)
Banggood warehouse is located in China. Hmmm… There is no active coupon in Banggood at this moment. Price? Hmmm, price is higher than average (which is 9.44 USD right now), so better set price alert down below…
Link: Banggood product page
Price: 14.99 USD
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k
Old coupons (from the oldest to the newest): BGT100Brush, 502C6B, vipupgrade8, BGDouble11HC, BGHHH2, BGBN2, BG2MFC, BGTMCB7, bGhOnEy10, BGXIAOMI811, BG777777, BG11025, BGAFF10OFF, BGSD58965, BGCC0704, BGTH459, BGHONEY2110, BGXM7707, BG10OFFAFF, BGwhielabel, 13f142, BG10OFFAFF1, BANGGOODSD, 6GSGIT, BGHealth06, 00c906, BG00c906, BGHB218, BGcc928b, BG75597c, BG844df1, BG55852d, BGb6c2c2, BG572079, BGcc81c0, BG3e4937, BG589b2e
(these coupons are old, so probably won’t work)
Aliexpress
Click here… to search for 3Pcs Xiaomi Mijia T100 Toothbrush Replacement Head on Aliexpress
Here are coupons that you can use.
Price history and price alert
Here is price history (for all/most countries) for 3Pcs Xiaomi Mijia T100 Toothbrush Replacement Head.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).