I'm inclined to agree, you might try emailing back and forth, or private messages on whatever social media platforms you both have. Writing letters could come off as sweet, but you've got to be careful in this day and age because, as Beerus said, it could also come off as creepy or overstepping by sending something to their physical address. Maybe take the time to chat privately on social media and see if they're open to the idea of you sending them a letter, as an expression of how you feel. If they're fine with it, then go for it!