It could be a "truth or dare" game. Do you dare to open the door, naked? But what happens next?
She is horny and she knows the pizza guy is good looking ... maybe she can seduce him.
She has no money, but she is hungry, so she orders a pizza. The girl is sure, if she opens the door naked, the pizza guy will be so surprised, that he will not charge her money. And even if he asks for money, she could pay him in another way. Maybe she allows him to touch her boobs or her pussy. Or maybe she even gives him a handjob or a blowjob ...
Just to add: I once saw a porn movie with Ron Jeremy. He knocks at a door and Susanne opens it. He asks for Cindy, but Susanne tells him, she is not at home Cindy took away Susannes boyfriend, so Susanne wants to get even. So she tells him, that she is much better and hotter then Cindy. She seduces him and they fuck. After they are finished, Ron keeps on asking for Cindy.
Susanne is surprised: "You probably just had the best sex of your life. Why do you still keep asking for this bitch Cindy?"
"Yeah, the sex was great. But you see, I am the pool boy and Cindy still owes me $50."