05.04 · Walkthrough
Usage Billing, Credits & Overages
Meter what a customer consumed and turn it into an invoice they trust, with credit blocks that expire and burn in a defined order.
Usage billing works best as an auditable pipeline: record consumption as facts, rate those events against the plan, apply credits from a separate ledger, then invoice any remaining overage. Trust comes from deterministic credit rules, idempotent metering, and a clear path from raw usage to final total.
What this lesson answers
- how should usage billing events be recorded
- how do expiring credits apply to invoices
- how are overages calculated in subscription billing
Notes
Usage billing starts by recording facts, not money. A meter should capture who used what, when it happened, how much was consumed, and an idempotency key so retries do not double-count. The billing system then turns those usage events into rated line items using the customer’s plan, prices, tiers, billing period, and any applicable credits. The invoice should be explainable from the raw usage through to the final amount owed.
Credits are prepaid or granted value that reduce future charges according to explicit rules. A credit block needs an amount, currency or unit type, effective date, expiry…
References
Common questions
- What should a usage meter store?
- A usage meter should store the customer, the thing consumed, the time it happened, the quantity, and an idempotency key. It should not store the final charge as the primary fact. Pricing can change by plan, tier, period, or credit state, so billing should rate recorded usage separately.
- Why not model credits as negative invoice lines?
- Negative invoice lines hide the lifecycle of the credit. A billing system needs to know when the credit was issued, what it can apply to, when it expires, how much remains, and why it was consumed. Keeping credits in their own ledger makes allocation, expiry, support queries, and finance reconciliation much clearer.
- How should credit burn order be defined?
- Credit burn order should be explicit and deterministic. Typical rules consume credits that expire soonest, promotional credits before paid credits, or restricted credits before general ones. The important property is that the same invoice, usage, and credit state always produce the same allocation and remaining balances.
Short definition: what is Usage Billing, Credits & Overages?