You can use either PS or P.S. – neither is incorrect – a quick internet search reveals that. Also, considering it comes from the Latin post scriptum which is no

You can use either PS or P.S. – neither is incorrect – a quick internet search reveals that. Also, considering it comes from the Latin post scriptum which is not capitalized and taking into consideration that writing in lower case is far more accepted these days writing p.s. is not a problem. But generally upper case is used. Compared to the loose/lose tectonic shift p.s. doesn't even register on my Richter scale. When in doubt, just look it up.