Overage
The charge for going past what your plan includes. First 10,000 calls are covered, call 10,001 costs extra.
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What it is
An overage is what a customer pays for the units past the allowance their plan already includes. Plan says 10,000 API calls a month, they make 13,000, so 3,000 calls get billed at the overage rate on the next invoice. It is the middle path between a hard cap (which breaks their product) and pure metered billing (which scares finance teams who want a predictable line item).
Design the rate deliberately. Overage priced at or below the marginal in-plan rate is an invitation to never upgrade; priced at 1.5x to 2x it nudges people onto the bigger plan, which is usually the actual goal. Decide the reset rule too: allowances almost always reset each period and do not roll over, and saying so on the pricing page avoids a support ticket per customer.
The gotcha is that overage is a surprise by construction. Nobody reads the limit, everybody reads the invoice. Alert at 80 and 100 percent of allowance, show the running overage amount in the dashboard while it accrues, and offer a mid-cycle upgrade that credits what they already spent. Also pick a policy for whether an overage-heavy month auto-upgrades the plan, and tell them before it happens, not after.
Ask AI for it
Add overage billing on top of the existing plan allowances. For each plan define an included quantity per metered unit and an overage rate per unit beyond it, reset at the start of every billing period with no rollover. During the period, compute max(0, usage minus included) and expose it as a live 'estimated overage' figure in the dashboard next to a progress bar of allowance consumed. Fire notification emails at 80 and 100 percent of allowance, and add the overage as a separate labeled line item on the invoice showing quantity, unit rate, and total, so the charge is never a surprise. Offer a one-click mid-cycle upgrade that prorates the plan change and waives the accrued overage.