For Lago, run a billing rehearsal with real-looking events: one customer changes plan mid-period, another exceeds a usage threshold, and a third receives a credit or refund. Compare the calculated invoice against an independently worked example. Then check event idempotency, late-arriving usage, tax requirements, and reconciliation with your payment provider.
Billing mistakes reach customers directly. Before replacing a system, confirm you can replay or correct events, explain every line item, export the ledger, and roll back a pricing change without losing the audit trail.
Akousa