A missed appointment is still charged at full cost. When you book a time, it means no one else can take it. If you don't give sufficient notice (at least 6 hours), I have no way to give this time to someone else who may really need help and support. They would miss out on counselling, and I would miss out on the income for that hour.
If cancelling at short notice (within 6 hours), the charge is 50%.
If I can fill the session, then no charge applies.