In the old days before personal computers, when people wanted to add something to a letter they had already ended, they would add a “postscript” (from Latin post scriptum, meaning “that which comes after the writing”). These postscripts were introduced with the label “PS” or “P.S.”
When they wanted to add something else after the postscript, it was labelled “PPS” for “post postscript.” But many people trying to follow this pattern today mistakenly write “PSS” instead.
Since modern technology makes it so easy to revise and add to texts, in most cases it’s better to just go back and insert the additional material at an appropriate point in the main body of the writing. “PSS” makes you look not only ignorant, but lazy.