I would wait after the Holidays to send your letter. If I were you I'd drive to the post office the day I was going to send it and put it in their mailbox where you can't retrieve it. I could see you going to the regular mailbox, putting it in, putting up the flag, going inside to sit down, and then running back to the mailbox to take it out again, lol.
As to the original question, I told both of my parents on the phone separately (they were in the midst of a divorce when I left home). I told mine on a whim really and I didn't care what their reaction was so much. I had a very cold "love me or don't but I'm doing this regardless" attitude about it. What can I say, I was a young country boy from NC who just moved to the outskirts of NYC to find himself. At that point I felt invisible. I wish I would have been more gentle about it, more forgiving towards their reactions, more mature, and understanding. Yet again, I was a kid then. I'm certain your way will have more tact.