Business letter writing is not a study of form and usages. It is rather a study of human nature and "how to use words so as to make people do things."
When writing to customers while it is important to have the core grammar elements and to appear professional it is important that your words are driving people to take action.
With the fast pace of communication we see people trying to go viral but while their message may be shared it is quickly forgotten and no true benefit comes from it.
When writing copy for customers the goal is for them to say, "where can I by that?" not "what a great tweet." Of course, the ideal would be to have the combination of the two.