I would advise you to wait a few days to send your letter. Cool off and reread the letter you wrote. If necessary, rewrite it then send it. I've found over the years that my first response often is a little overbearing...but thats just me.
I do think that companies need to know how their employees are responding to customers- both bad and good. They always take things more seriously if the letter is not too emotional and is factual. Good luck and I'm sorry you had such a bad experience.