You can pull an invoice directly from a calendar appointment. If you click on the appointment for which you would like to do this, a window will open on the right hand side which where you can see "Create invoice" as below:
If you click "Create invoice" you will be led to the below page where you will be able to edit the invoice as required, you should have the appointment name and price automatically pulled in to this invoice:
To edit the price and quantity of the appointment invoice, click into the invoice item and amend as necessary. Ensure you click the tick when you are finished:
If you would like to mark this appointment as billed then you can click on the appointment, a window will open on the right hand side, click on the "..." in the top right hand corner and then click "Mark as billed":
A bill note will appear on the appointment as below:
If for whatever reason you want to mark this appointment as not billed, click on it again, click "..." and then click "Mark as not billed":
This will automatically remove the bill note from the appointment.