How much is postage and how cheap could it be? (Depending on area/connections/currency etc)
If people could write a letter from their computer, and email it to someone else to send (Or post it somewhere), you'd probably get 5x as many letters. I've got a lot of friends that started when they were 6 years old and aren't comfortable mailing letters yet. After that, I have even more lazy friends that could write a letter but will never leave their room to send it. I know sending an email isn't as meaningful, but the content will be just as good, and the large quantity would be nice.
I know one person volunteered to send it for others, but unsure if he's prepared to send hundreds if he gets them.
If someone could get something together for something like that, might be a huge turnout.
We could even go to the forums and get people to post their letters there.
Just an idea, albeit an expensive one.