You are thinking way too much about this. Just send them an email saying that after careful considerations, you will not be matriculating after all. That's all you need to do.
Think about this way, if the school were to reject you, they wouldn't sit there for days and think about how to write a nice letter to reject you. They would just send you the same old BS rejection letter that they've been sending all the other rejected applicants. So there's no point for you to go out of your way and be extra "considerate".