Alex,
My only marketing is done whilst and by cleaning, but It seems to me the two-letter approach would be a waste of time. In effect the first letter would be "Do you want me to try to sell you something" question.
If the prospective customer is already reading something, you should be selling then and there. A follow-up phone call within two days of receipt of the letter , whilst your many benefits are fresh in mind, should pay dividends.
John.