Regular Trying to Lowball me 😡

Customer I see fairly often is trying to chop $200 off of what they typically donate for a session and is starting to get more demanding...

Not sure how to handle the situation as I don't want to lose the regular, but I'm also terrible at conflict resolution. I'm supposed to see them tonight and have made it very clear that I won't go below the regular price, but they keep flip flopping on me. Any ideas of what I should do/how to handle the situation?