If you cannot explain what happened on your end to make things go wrong, then explain the quality controls that your organization has put in place and say that problems are rare. Simply express regret that the writer experienced an unfortunate exception. Simply state that the customer's doing X caused Y to happen, and so the writer's request cannot be fulfilled. Your purpose in both kinds of adjustment letters is not merely to deliver your message. Instead, in both negative and positive responses, your purpose is to maintain good relations with the writer of the complaint.
Continued business is the goal.
He has recently bought a new house and commutes 250 miles to work in a car purchased in 1993. Well, that's hard to say exactly. Generally, sales letters are written for just the kind of person who would do what the writer wants. If the addressee is being asked to donate to his alma mater, then the writer lovingly describes the reputation, the tradition, and the current achievements of the school in emotionally charged language. The writer assumes that the reader is the kind of person who is predisposed to do what the writer wants.