In this case, is not urgent, and to communicate the same story to get a lot. Communicating with mail, surely less than the cost of other types of communications and waste less time to travel.
In non-urgent cases, and the need to communicate the same to as many recipients. Communication with the mail would cost less than other types of communication, and waste less time traveling.
In case of not urgent and the need to communicate the same thing to the recipient a lot. Communication with the letter would cost less than other types of communication and waste less than travel.