Andy- you're adding the wrong things. You don't add what they paid in the end to what the delivery boy kept to get what they paid originally. While it might be appealing, that isn't the formula. There are issues it doesn't take into account (change, price of pizza)

THe question is does the money paid out equal money received? There are two ways of looking at that question.

Money originally paid out is $30 -- which equals $3 in change, $2 in tip, $25 for pizza.

Overall money paid out is $27 -- which equals $25 for pizza plus $2 tip.

You can't mix apples and oranges.