*This is constructive criticism*

If I was the one who received this letter, and saw this as the second sentence:

"Not only are your staff completely and utterly incompetent, they just do NOT want to help when someone asks for help."

I would have deleted the email/thrown out the letter right there. As angry with them as you are, when you start off a letter by insulting someone, the first thing that person is going to do is NOT want to help you. Corporate or not.

That's just my 2 cents. The rest of the letter is written pretty well however.