She probably thinking there will be interaction with / from the girlfriend. 3 way snowball
Even if the girlfriend (or anyone else) was just to watch they would expect extra.
When I’ve organised FFM with 2 SPs they both still expect their hourly rate, plus some travel (lost time) money. Some expect more for girl on girl. What happened to “you are paying for my time”.
Was this an outcall?
Anything extra / away from a normal punt tends to cost extra in puntingland.
Get a regular and she may let her join in as a freebie to keep your business, but there may be awkward familiarity difficult to explain to the GF
Edit: what does your lass look like. How she getting on with her craving for extra cock?