The letter N (window becomes widow, door becomes donor).

Teeth

Pull a fruit from the “Apples and Oranges" crate. Let’s say you pull an apple out. That crate is full of apples. That means that the crate labeled Oranges has to be Apples & Oranges because apples is already taken, and it's mislabeled. That leaves the remaining crate to be oranges.

Envelope

The $2 shouldn't be added to the $27 — it's already part of the $27 ($25 for the rooms, $2 for the bellhop). The remaining $3 to get to $30 is the singles given to the 3 guests.