The balance, settled at the door
The balance is the part of the fee not paid ahead: half on a booking with a deposit, all of it on a returning guest's same-day hour or two in the city.
Every booking is paid in one of two ways: half ahead and half at the door, or all of it at the door. The part paid at the door is the balance.
How much
On a booking that carries a deposit, the balance is the other half. Ninety minutes with one therapist is EUR 270, so EUR 135 goes ahead and EUR 135 is paid on arrival. On a returning guest's same-day request for one or two hours at a city address, there is no deposit, and the balance is the whole fee: EUR 180 for an hour, EUR 360 for two.
How
Cash in euros is the plain route. Cash in another major currency is accepted as well, provided the exact amount in that currency has been confirmed to you in writing in the thread before she sets out. No price is ever published in anything but euros.
When
On arrival, at the start of the visit. Settling it first means nothing about money needs to be thought about for the rest of the session.
A long-stay habit
Guests who book every week or two often keep the right amount set aside in an envelope in a drawer, so that an evening decided on the day does not also involve a trip to a cash machine in the rain.