Hi Odga~ I follow your postings as we are in simular places, (D, yet?...)
I have written a couple letters in the past. They were marginally effective. H seemed to appreciate them, and we talked a bit. But in the grand scheme of things we are still D, although have casual contact much like you and C.
Some things that come to my mind: What do you want to accomplish? What is your message and what would you envision the outcome to be?
One of the heartfelt letters I wrote to H I wrote on really nice paper with quality pen, by hand. There was a big delay in H getting the one I wrote in the mail and he requested me to email him the letter prior to him getting it. Once he did get the real one he mentioned how much better it came accross in my own hand.