Journey time, agreed before any airport start is fixed
Journey time is how long she needs to reach a room at Schiphol, Schiphol-Rijk or Schiphol-Oost. It is agreed with you first, and the start is fixed after.
Journey time is the one element of an airport booking that a city booking does not have. It is the time a therapist needs to travel from where she sets out to the room you are staying in at Schiphol, Schiphol-Rijk or Schiphol-Oost, and it is agreed with you for each booking before any start is confirmed.
Why it is agreed, not quoted
The airport lies beyond the ring road, and the journey there depends on the hour, on roadworks, on the weather and on where she is setting out from. A fixed figure quoted in advance would sometimes be wrong. Agreeing the time per booking means the start you are given is one that will hold.
What goes into it
The road journey itself, plus the time it takes to get from the entrance of your building to your door. At night that second part can be longer than expected: a front door that locks after midnight, a desk that phones up before admitting a visitor, a lift that needs a key card, a business park where the lit entrance is at the back. Mention these in the thread, and they are included.
How the agreement runs
You give the address, the time you expect to be ready and the length you want. The house agrees with the therapist when she can set out and how long the journey will take. The thread proposes a start; you accept it or ask for another. Half the fee then goes ahead, and the start is confirmed.
What it does not change
Journey time changes the start, never the figure. An hour at an airport room costs EUR 180, as it does in the city. The arrival window entry describes how the city works instead, and the journey time guide gives a full worked example.