Part IV: Recurring, Subscriptions & Complex Billing
Where payments meet business models.
Audience: Operators → Architects
This Part explores what happens when a payment isn't a transaction but a relationship. One-time checkout is a solved problem; charging the same customer every month — without them present, without their card expiring underneath you, and for an amount that may change with usage — is where billing engineering actually lives.
The three chapters read as one arc. Chapter 15 covers what happens when a recurring charge fails — decline taxonomy, retry strategy, and the dunning machinery that recovers revenue. Chapter 16 steps back to ask how the merchant earned the right to charge at all: stored credentials, CIT/MIT classification, SCA, and mandates. Chapter 17 then tackles the hardest question — figuring out what to charge — through metering, rating, invoicing, proration, and tax.