'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 Wikipedia page? Is it OK to abbreviate it as ps.?
Update: It seems Dictionary.com may have been wrong with its etymological explanation of the modern English word. Nevertheless, given that the modern English 'postscript' is one single word, shouldn't the abbreviation be able to be updated?