If you exceed your session commitment and Buffer Session allowance, you'll see a new colour on the Session Consumption chart in your company's Billing tab. This new colour tracks Flex Usage.
Flex Usage is essentially a pay-per use plan to tide you over until your next term begins. For each month of Flex Usage, you'll be billed on a tiered model based on sessions used. As soon as your account switches to Flex Usage, a new Flex Usage section will appear on the Billing tab. Here, you can request invoices for each month billed under the flex plan.