Postscript (P.S.) Definition and Examples in Writing

A postscript is a brief message appended to the end of a letter (following the signature) or other text. A postscript is usually introduced by the letters P.S.

In certain types of business letters (in particular, sales promotion letters), postscripts are commonly used to make a final persuasive pitch or offer an additional incentive to a potential customer.

Etymology
From the Latin post scriptum, "written afterward"


read examples and observations here