You can allow clients to temporarily extend their bandwidth limit by purchasing bandwidth packages, this can be enabled from Client Functions tab in product configuration.
To set it up, enable "Additional Bandwidth" function.
Click on pencil next to Additional Bandwidth feature, proceed to Options tab. Here you can setup additional bandwidth packages offered to customer:
After that your clients will be able to to purchase additional bandwidth from Billing and Resources (Cloud Hosting only) tab for related Proxmox service in client area: