The balance is the half of the fee that is not sent ahead, handed over when she arrives, before the session begins rather than after it ends.
Every booking that carries a deposit leaves a second half outstanding, and that half is the balance. It is paid at the door when she arrives, before anything starts, which keeps the whole question of money inside the first two minutes of the visit rather than hanging over the last ten.
Two hours for one guest is EUR 360, so EUR 180 travels ahead and EUR 180 is handed over on arrival. A night at EUR 1,250 splits into EUR 625 and EUR 625. Two therapists for two hours is EUR 720, halved into EUR 360 and EUR 360. Each of those figures is printed in the profiles on the roster. A returning client taking a same-day hour inside the city sends nothing in advance and pays the whole EUR 180 at the door, which is the one booking where balance and fee are the same thing.
Cash has exactly one place in this arrangement and this is it. It cannot serve as a deposit, which is settled before she travels, but it is entirely welcome in the room. Prices here are quoted in euro throughout. A balance paid in cash in another major currency can be arranged, and the exact sum in that currency is confirmed to you in writing before she sets out, so nobody is doing arithmetic on a phone at three in the morning.
The practical advice is small and it saves the only awkward moment a late booking tends to produce. Count it out before she arrives and put it somewhere you are not going to have to search for. A guest hunting through a jacket on a chair while someone stands in a doorway at two in the morning is the one scene worth avoiding, and it takes thirty seconds to avoid it. Nothing is charged on top of the fee at any point, and there is nothing further to settle when the hour ends.
Two lines settle it: where the room is, and what time you want someone at the door. The answer comes back with a name, an arrival window and the whole figure, whatever the clock says.