Cancelling costs only the part of the fee already earned when you write, on a three-step notice scale; the fourteen-day withdrawal right does not arise here.
Notice decides it. A cancellation costs the part of the fee the house has already earned by the moment your message arrives, and nothing is ever charged on top of that part.
Two rules sit above all three. When another client takes the hour you gave up, the whole deposit goes back to you. When you can show the house lost less than the sum it kept, the lower figure is all it keeps.
A booking may change date once at no cost, provided the request reaches the thread more than twenty-four hours ahead and the new date falls inside fourteen days. The deposit crosses over whole. A later move, or one asked inside the last day, is read on the same three steps as a cancellation sent at that minute.
The choice passes to you: the deposit back in full, or held against a later date for as long as you need to pick one. If the therapist you booked cannot come, you hear so before she would have set out, and you either choose someone else from the roster or take the money back.
End a session early and the time reserved for you stays payable, since the hour was held for you and its unused minutes cannot go to anyone else. If a limit she has set is pressed, she ends the visit there and then; she has come through your door by that point, the hour was kept for you and cannot now go elsewhere, so the fee for the reserved time stays due, and no further sum follows it.
A booking agreed by message is a distance contract. The rules for it sit in Book 6 of the Dutch Civil Code, articles 6:230m to 6:230z. Article 6:230p takes certain contracts out of the fourteen-day withdrawal right, among them services related to leisure activities where the contract fixes a specific date or period. A booking here names an hour on a day, the kind of contract that exception exists for, so the right does not arise. For a same-day session the thread also asks you, before she sets out, to confirm you want it performed at once and understand the right will not apply once it has been.
The complete position is on the booking terms; who pays half, and how, is under deposit.