Invoices normally include line items for the email marketing plan and disk space plan used, but invoices can also include line items for overage. Overage occurs when contacts are added/imported beyond the current email marketing plan or resources are uploaded beyond the current disk space plan.
For example, if the account has the free 500 contact plan and their total contacts surpass 500 to become 501 or greater, the account's invoice for the next billing cycle will include a "Email Marketing Plan Overage" line item. This line item will include the email marketing plan that was used based on the actual number of contacts the account had for the previous billing cycle and the price difference between the original plan and the actual plan will be reflected in the "Amount" column.
For more information on how service is billed.