Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in Hong Kong. I found coupon for you. Check this out.
Link: Banggood product page
Coupon (359.99 USD): BG238a4d
(CouponsFromChina.com tip: coupon code has to be applied in shop’s 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): BGfcfdae, BG0bdb00, BG1df224, BGAUGUST8, BGeb3b5e, BG920b85, BGBFDAY8
(these codes were not working in latest coupon update, but you can check them out anyway)
Price history and price alert
Price history (for all/most countries) for Beelink SER3 Ryzen 7 3750H 16/500GB Mini PC is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).