Add more postscripts by adding a 'P' to “PS.” The “post” in “postscript” means 'after' in Latin (and “script” means 'writing'), and so when you add another 'P' to a “PS,” you're writing an “after-after-writing.” PPS (and PPPs and PPPPS) is best used in informal correspondence to add a little bit of humor or cheekiness.
Ing to the Cambridge Dictionary, the correct American format is 'P.S.' with periods after each letter. In contrast, the Chicago Manual of Style advocates PS without periods.