with gluonhq charm down, how to use InAppBillingService for “discussing” with Google Play

Clash Royale CLAN TAG#URR8PPPwith gluonhq charm down, how to use InAppBillingService for “discussing” with Google Play
I want to commercialize my Javafx product in Googe Play store. To do that, I included the gluon plugin :in-app-billing. But, for using this service, I only found this small example from http://docs.gluonhq.com/charm/javadoc/4.4.1/com/gluonhq/charm/down/plugins/InAppBillingService.html
This example does not accurately explain the billing and purchase mechanisms with Google Play. Where can I get a Java code examples that provide the mechanisms as it is explained in the Android studio environment with https://stuff.mit.edu/afs/sipb/project/android/docs/training/in-app-billing/index.html , for instance?
That would cover these questions:
- how to initiate a connection with Google Play with keys, ....
- how to purchase a product
- how to know if the product was already bought, before?
- ....
Thanks
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