An extra billing method that can be applied to students billed at an hourly rate from the service list or the student's profile is the cost premium. This allows you to add an additional charge per session, meaning that it is not affected by the duration of the session. This is most commonly used for charging students that require a teacher to travel a longer distance to meet. It's also possible to add a wage premium to the teacher's wage for each lesson.
You can read more here: Cost & Wage Premiums