Thanks for your reply @KarlOffenberger .
Here is an explanation of subsequent steps
In my custom storefront User chose to buy a service. (The product is NOT marked as Physical product)
I create a check-out(123)
redirect the user to Shopify Payment page.
The user completes the transaction.
I need to know(on custom storefront) that the checkout 123 was converted to an order by successful payment, so I can automatically enable the service in checkout for the user.
I want to avoid manual intervention at this stage
Hope that helps.
PS: I like the signature line “I turn coffee in to code - since 1998”