After creating an add-on, you can use it to generate a one-off invoice that will be billed instantly.
A one-off invoice may include multiple fees (i.e. multiple add-ons). You can add a custom description and set a specific price for each add-on, either through the user interface or the API.
The endpoint to apply add-ons is deprecated and will be removed on September 1st, 2023. It has been replaced with the endpoint to create one-off invoices.