Aliexpress
Click here… to search for SANTAFELL AC220V 1-7L/min Standard Version Oxygen Concentrator on Aliexpress
Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
Coupon (149.99 USD): BGd963b1
Coupon (272.99 USD): BGGutscheine
Coupon (275.99 USD): BG8ReviTrage
Coupon (275.99 USD): BGcoupon8off
(CouponsFromChina.com tip: paste coupon code in your cart and click Apply 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): 2022ACTIESNL, Terence, Tabby20, 2203ActiesNL, BG1efeb0, DealBGimpact, Cuponation9, 9OFFSUMMER, BgGoodie9
(I don’t think these’re gonna work, but you can try ’em out anyway)
Price history and price alert
Here is price history (for all/most countries) for SANTAFELL AC220V 1-7L/min Standard Version Oxygen Concentrator.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).