with no explanation I could see myself going either way on that; but it seems to me that you could give an explanation in the body of the letter, and like--I at least trust you enough that if you wrote "no but really it's chill tho", I'd believe you. because letters are anachronistic, it seems unlikely to me that there could be a general etiquette on the subject, so probably it depends on the person in question and their personal etiquette. you could also, if it wouldn't spoil the surprise too much, ask them beforehand how they'd feel about it--but, ahaha, that's a very "me" answer.