I think the transition from fantasy to reality is very subtly conveyed in a way I can get behind. We definitely start parasocial, but when she starts becoming dissatisfied with the fantasty and more interested in and curious about the real person she's delivering to... ionno. That the ex-idol is dressing up for her as an idol and isn't showing up as honestly as she was at the start is honestly coming across as more parasocial to me - because she's recreating the environment that they started in believing that's what the delivery girl really wants from her.
Ionno y'all I thought it was cute. It's a shame it's a oneshot. I kinda feel like you could easily write a whole story about this kind of thing.
I agree with you there, it feels like they were both somewhat conscious about walking the line of parasocial vs seeing the human behind the role. when the delivery girl says "i love the micchu that keeps secrets, but i want to know more about saotome-san" its absolutely intentional that she uses her idol vs real name. And i think it represents a step beyond the parasocial, and sets the stage for them meeting irl as equals. Even if the way they got there was kinda rocky.