Sure I suppose that's a risk. But then they'd be in breach of the contract since the letter definitely doesn't say to cancel the account, it says that this is notice given to all extant notice clauses that could result in additional billing, renewal, and so on. It's worded well I'm paraphrasing.
As a practical matter though I don't think there's much to worry about. These guys don't have a process to field letters like this.
The joy comes from the elegance of it all. If they want to play a game of exploiting the fact that people don't read the fine print then they should be prepared for a fair contest.
But isn't there a risk that if the company that gets your certified mail, reads it, and then cancels your account?