Once the credit limit of your current plan is exhausted, all email requests count as an 'Overage'. Depending on your subscription plan, the overage cost per email varies.
Overage fees, if any, gets charged when
- the overage value reaches $100 or more, or
- In the invoice of the next billing cycle.
Whichever comes first.
Upgrading your account does not absorb overage charges already incurred. In case you place an upgrade request when there is an overage in your account, then the overage is first calculated based on your current plan and post that the upgrade request is processed.
On upgrade, you are immediately charged the accumulated overage cost and new subscription cost.