Wikipedia says: A postscript may be a sentence, a paragraph, or occasionally many paragraphs added to, often hastily and incidentally, after the signature of a letter or (sometimes) the main body ...
'Postscript' is a single word in modern English, and Dictionary.com states that it's even based on a single Latin word, postscrīptum. So, why do some abbreviate it to p.s. (or P.S.), as on this ...
Somewhere in the craggy quagmire of my memory, I seem to recall that the nuns of my grade school days taught me that a P.S. (post script) is followed by a colon, i.e. P.S.: Alas, the periods after ...
There was a Postscript section in my English Course-book, which simply teaches the colloquial English phrases as in "Give me a lift", "Let me have a look", "There is no point", etc. But "postscript" ...