[Edmond]

I would second this advice :)

I fumbled and bumbled my way to this realization while trying to build a billing system intended to tolerate all sorts of invoicing and reversal scenarios.

More precisely, it doesn't matter what the nature of the billing is, ie recurring (monthly, quarterly..etc) vs one-time; You want to build a system around invoicing for discrete items and resolving those invoices against various criteria (payment received, credits issued, cancelled plans...etc).