It didn't make any sense, and was always a really jarring moment for me. It was a jarring moment for you, too, but the only thing you remember about the movie was Tom Petty's cameo. Trust me. At the time, you were like, "What's up with that girl?"
WELL. It turns out that that girl never really did much else (she was in a video short called Lazy Teenage Superheroes in 2010, and it was her only role since "Ponytail"). And prior to The Postman, she had appeared in only two other movies: The Baby-Sitters Club, and Dances with Wolves.
You could be forgiven for perhaps thinking that the young actress had impressed Costner on Dances with Wolves, and that he remembered her and invited her for a small role in The Postman.
She's his daughter.
Delightful experiments are happening in the kitchen, and I'll update you as soon as they have come to fruition.
Tomorrow, gentle reader, ADVENTURE!! :)