No problem! A provider can adjust their billing preferences under the "Account" tab. You can choose one or both of either per-appointment or subscription billing methods for each of your clients.
"Subscription" is where you charge a flat rate for a period of time, and can add or subtract lessons as you see fit. While you do not have to add a charge to lessons added to a subscription after it is paid for, you can if you wish. Just make sure you have activated both billing options in your account.
"Per appointment" or "Hourly" billing does allow you to set a one time appointment with a client each month, and you can even set that appointment to recur each month. This will automatically bill your client at the same time each month until you or they begin the cancelation process.