I agree, I don't like it either.

It is generally felt that negotiating brings unease to the encounter.
It's probably best to just discreetly hand over the listed amount for the required time.
Everyone is then happy.

Always discuss your needs, such as possible extra like Anal. Sometimes favourites can be extra.
Better to be happy that she is giving what you want before handing over cash, then everyone is happy. Girls aren't mind readers, they only pretend they are