srgause is correct it's PS, PPS...PPPPPPPPPPPPPPPPPPPPPPS. ;)
In my experience, any current use of PPS, PPPS, etc. is usually made in a casual, humorous way, and that's how it should be taken, as well.
Why add extra Ps and not Ss? Because P.S. stands for "post-script" or "post scriptum" (Latin), meaning "after the script", so the extra "P" in P.P.S. means "AFTER the after-script".