Simple subscription handling

A common use of Billwerk+ Optimize and/or Billwerk+ Transform is to have customers in the own system and customer subscriptions in Billwerk+, where at any time a customer will have at most one active subscription.

This simple use case demonstrates how Billwerk+ can be used for signup, access control and subscription management without storing any data on the own system except the customer with a unique id. At some point, the solution could be extended to keep data on the own system for quick access. This could be done by synchronizing data between Billwerk+ and an own system using Webhooks.