Isn't she there to get cosplay clothes? Maybe it's just me, but if I want to buy something from a craftsman, the first thing I do is not to try to blackmail them. I try to strike a normal business deal with them, one that would satisfy both sides. I imagine 99.99% of people in the world act like that. That insane girl, however, seems to think that blackmailing is the norm. At this point Gojou should be the one calling the police, but I fear his timidity and shyness will prevent that.
At least now we know there won't be any romantic triangles. There's no way for Gojou to fall for an ugly blackmailer.