Balance on arrival, the second half
The balance is whatever remains of the fee once a deposit has been paid, normally the other half, and it is settled at the door when she arrives.
On every booking that carries a deposit, the fee is split in two. The first half goes before she sets out; the second is the balance, and it changes hands at the door.
What it looks like in practice
Two hours booked the week before: one half of the two hour fee ahead, the matching half on arrival. Three hours on the day: half ahead because of the length, half on arrival. The overnight: half of the block price each way.
Currency
Every fee the house quotes is in euro. A balance in cash in another major currency is welcome, at an exact figure in that currency confirmed in the thread before she leaves, so nothing is worked out on the doorstep.
A practical note for regulars
Guests used to paying at the door in full sometimes forget, on a larger booking, that half has already gone. The thread states the balance before she sets out. Have that figure ready rather than the whole fee.