This can't be repeated enough.
It's been a lot of years since I served, but I can still remember how good it felt getting mail. Believe me, your service person will read and re-read your letters often.
There's nothing more disappointing than a mail call that doesn't produce a letter for you. :-(
The Texas Army National Guard bought several boxes of Christmas cards and had full time and weekenders sign each card. We are sending several hundred cards from the central Texas area.