Actually I already faxed a copy to our accountant (best money we ever spent!) and she said the amount in dispute was claimed so she is writing them a letter about it. I do fine it interesting that the letter is dated the 7th and they give you a flat 30 days to settle up or face the consequences.
There was a time when this kind of thing would have really upset me but now I just figure, "what have I got that you want so bad, well you can just have it because in the end it's just stuff..."