I have seen it done both ways, Up Charging or a Flat Rate.

I charge an amount for your time that covers most of the activities folks ask for when they call.  The very occational outside the box activity is negiotiated before they get an appointment.  I always ask what activities a client is seeking.  There are extreme things I just don't do professionally.  I refer them to friends that do, who I know.  Sometimes I negiotiate a session that includes another provider.

Up Charging happens when they insist on adding something not included.  I avoid this.  I stick to the negiotiated scene. If a client has negiotiated a scene, they are now trying to break the limits set.  If you cross that line, it opens the can of worms that all the other limits set are not hard limits.

There is always that moment when you get asked to do something you are just not fond of in beginning negiotiations.  When i was starting out, a professional Domme told me that at that moment you ask the amount that if you did it, you would be good, you'd be excited to do the session with passion.  If a clients wants that activity and they want you, they pay it.

I always make it clear what is included in my Flat Rate and what isn't.  My experience is that folks get upset when they assume some activity is included and it isn't.  Failed Exprectations are a bummer.

For me, relationships are built over time.  There are activities I do with folks I know better because I have seen them several times, and I have a foundation built that i trust them more.  Example:  Being bound helpless in rope and struggling.  Hot time.  I would not do this with a stranger.  A submissive's first job is to protect the toy for further use.