I have an app that delivers up services from my back-end server to the app.
The Free version provides some functionality, for free, but to get Full access, the user should buy the fully paid version of the app.

I am trying to find out how to be notified by the Play Store when someone buys my app so I can have that person then enter for example their receipt number, to be recorded in my back-end, to verify (match) the info from Play, so I can verify the app has been purchased and thus they get full access.

There is quite a lot of information around about how to record and verify in-app purchases, for

example ;

https://developer.android.com/google/play/billing/backend
and 
https://developers.google.com/android-publisher#subscriptions

but what I need is simply to know when an app is purchased as a one-time.

I see there is something called Real-time developer notifications, but I am having trouble finding good info on what this does in full and specifically, how to implement it.

I have also seen this ;
Get all notifications for subscriptions and one-time products – receive notifications for all subscription and voided purchase events. You’ll also receive one-time product purchase events, such as ONE_TIME_PRODUCT_PURCHASED

But I cannot find out how to integrate that to the server back-end to see this data.

Appreciate any guidance or links.