It's bad to flake out at the last moment.
Cody has a birthday party he RSVP'd for, but a concert by a band he loves is on the same day.
Cody wants to go to the concert.
Cody informs the birthday hosts before hand that he cannot make it and apologizes.
Cody tells the birthday hosts he cannot come the day of the party.
The birthday hosts will accept that Cody cannot come and reduce the party materials.
The birthday hosts will think Cody is rude and have excess party materials.