Indeed, you'll often find it abbreviated as such in the US. But The Chicago Manual of Style favors PS, without the periods. ... The safest bet is to capitalize the P and S (use periods after each letter if that's your preference), and leave out any trailing punctuation.

How do I write a Post Scriptum?

Add more postscripts by adding a 'P' to “P.S.” The “post” in “postscript” is a Latin word meaning 'after' (“script” means 'writing' in Latin), and so when you add another 'P' to a “P.S.'”, you are creating an “after-after-writing” to form P.P.S.

What should I put after PS?

This, of course, is because “PS” stands for “postscript”. This comes from the Latin “post scriptum” (sometimes written “postscriptum”), which translates to “written after”, or more to the point, “what comes after the writing”.