To properly bill for an Appointment we want to ensure that Invoices have a blue hyperlinked Appointment. When we properly bill for an Appointment the Appointment will be marked on the calendar with a green indicator that says, "Billed."
When you create an Invoice from the Client's Ledger you will want to ensure that the blue linked appointment appears. Anytime you're billing for a session you do not want to add an invoice item and create it manually. If you create it manually it does not link the appointment on the calendar and therefore will not show the "Billed" indicator on the calendar appointment.
Here is more information on how to create an invoice.
If you would like to correct an invoice so that it is linked to a calendar appointment you will want to go to the client's ledger > "Select an Action" > "View Details" > Click the "X" to the right of the line item > Use the "Recent 30 Days", "All Past" or "All" filter > Click the "X" to the right of any line items that you do not need on your invoice > Save.