When you want other persons to receive a copy of the letter you have written to the addressee, the names of these persons (arranged in order of importance or alphabetically) should be typed after "cc" just below the reference initials or enclosure notation (if any)
Word as "special Delivery" "Registered" "By Hand" or "Confidential". When applicable, may be typed either below the carbon-copy notation (or whatever is the last notation) or on the second line below the date and on the envelope.
It Should not be used to indicate that you have forgotten to say something. However, to street an important item briefly or to include a personal brief message unrelated to the letter, a postscript (P.S.) may be used following all other closing notations.