Dear Elder is an excellent service to get messages to your missionary quickly. You write a message at the site and send it. Dear Elder will then e-mail those messages to your missionary’s respective mission. The Elders in the office will print them out, put them in an envelope, and deliver them to missionary.
It’s pretty convenient. The only draw back that I noticed on my mission is that because the Elders in the mission office are the ones who stuff the envelopes, there isn’t much privacy. So, don’t plan on writing a Dear John to your missionary using Dear Elder, unless you want the entire mission to know.