The balance, paid once she is at your door
The balance is whatever part of the fee has not gone ahead as a deposit, paid at the door when she arrives, in euro or in cash in another major currency.
There are two ways a booking gets paid. Either half goes ahead as a deposit and the other half is the balance, or, for a returning guest's same-day hour or two at a city address, the whole fee is the balance. In both cases the balance is settled when she arrives, before the session begins.
How to pay it
Every rate this house quotes, on a profile or in the booking thread, is stated in euro. The balance can be paid in euro, and cash in another major currency is also accepted, with the exact amount in that currency confirmed to you in writing before she sets out, so there is no calculation to do at the door. Having it ready in an envelope or on the table saves a minute at the start of the evening.
What it never includes
There is no charge for her journey anywhere in central Amsterdam, no surcharge for the evening, the weekend or the hour, and nothing is added for a building without a lift. The balance is simply the rest of the figure printed on her profile for the length you booked.
Why it is not paid in advance
Keeping part of the fee for the door means the arrangement is shared: the deposit commits you, and the balance is only due once she is there. The deposit entry explains which bookings carry a first half, and the roster shows the figures for every length.