Austin Parker,
1234, West 67 Street,
Carlisle, MA 01741,
(123)-456 7890
[Subject: Normally bold, summarizes the intention of the letter] -Optional-
Dear Austin Parker,
According to the telephonic talk with you, I realize that you have been unable to pay your earlier due of $370 against a total due of $2000. May I tell you that you will have to immediately pay the outstanding amount as soon as possible, and from now on you will have to pay cash for all your transactions.
Sincerely,
Adalgisa