You can specify an amount you’ll charge if the client misses their appointment or cancels. To do this, enter the dollar amount you wish to charge and select the number of hours (24-72 hrs) they must give you in advance before the charge occurs.

When you mark an appointment missed it will charge the dollar amount you had previoulsy specified in the Missed Appointment Fee box on their billing settings page when you create and save the missed appointment note. You can override the fee by removing or changing it when you create a missed appointment note.
If you leave the Missed Appointment Fee box blank, the client will not get charged regardless of the setting Charge if appointment isn't cancelled (XX hrs) in advance. If you decide to charge the client anyway, you can add the fee to the missed appointment note and an invoice will be created for that amount on a one time basis.
Appointments that are cancelled more than 24-72 hours in advance (depending on what you set from the drop down) won't be charged. Appointments cancelled in less than 24 hours will be charged if the Missed Appointment Fee box is filled in.
If a client cancels an appointment using the Client Portal the same rules apply.