> in my use case I only need to handle a specific subset of the complexity
Thing is billing money leaves very little room for error and is regulated.
From the start you'll need to understand all the dos and don't of personal info handling, the cycles and lifetime of the different billing options, cashback thresholds, support idempotency, sane DB transaction management with models that are adapted to the task, invoicing and exposing that to your client etc.
It's not everything at once, but at least half of it just for your first workable implementation.
Thing is billing money leaves very little room for error and is regulated.
From the start you'll need to understand all the dos and don't of personal info handling, the cycles and lifetime of the different billing options, cashback thresholds, support idempotency, sane DB transaction management with models that are adapted to the task, invoicing and exposing that to your client etc.
It's not everything at once, but at least half of it just for your first workable implementation.