The problem
Northwind's billing logic had accreted six pricing models, two acquisition migrations, and a backlog of manual finance corrections. The finance team was spending three days a quarter reconciling Stripe to the database.
What we did
We designed an append-only ledger as the source of truth, modelled every pricing case explicitly, and built a back-fill that reconciled four years of history against Stripe. Then we cut over with a dual-write period and verified parity.
The result
Billing tickets dropped 71%. Reconciliation went from a three-day quarterly project to a 12-minute automated run.