Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
Coupon (27.87 USD): BGXMHJ
Coupon (30.59 USD): AFfbG10
Coupon (30.59 USD): band10
(CouponsFromChina.com tip: put item in your cart and apply coupon code there 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): F8AAF1, f8aaf1, TTxBG10, e2bcea, 502C6B, bghey, BG3CSUM, BGHEYPL, BGsaldihey, bghj, BGBFAFF2019
(sometimes old coupons work, so you can check them out (this site checks them on every price update))
Price history and price alert
Price history (for all/most countries) for Xiaomi Hey Plus B1800 Smart Watch is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).
Thank you!
Coupon 1 ($31.79): BGHEYPL cannot be used. It said selected countries only. I’ve tried Canada and the States but neither worked.
Sorry. You can create price alert though. Maybe there will be new coupon that will work in Canada