Events that you can "listen" for in the backend; PURCHASES

I need to update the Inventory of a Product when a user makes a purchsae. I have 2 product listings with differnt prices. One of the products is listed @ a discount for paid subcribers. When either product is purchsed I need to update the others to '0'. I currently, have a 'events.jsw' for the backed but I am not sure which functions to call. Also, where would I put the listener code?? In the 'Thank You' Page. On Velo's ref page I was looking @ 'onCheckoutCompleted' & 'onCartCompleted' but I am lost at the moment. https://www.wix.com/velo/reference/wix-ecom-backend/events/oncheckoutcompleted