Aliexpress
Click here… to search for Beelink SER3 AMD Ryzen 7 3750H 16/512GB Mini PC on Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in Hong Kong. Well, there is one coupon available right now.
Link: Banggood product page
Coupon (339.99 USD): BGbbbfcf
(CouponsFromChina.com tip: you should apply the coupon in your cart 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): BG01616b, BG2a78b1, BG275071, BGe06c69
(probably won’t work, but you can try them out)
Price history and price alert
Price history (for all/most countries) for Beelink SER3 AMD Ryzen 7 3750H 16/512GB Mini PC is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).