Answer:
"P.P.S." is what I think you mean, not P.S.S. If you use P.S., which would be used for an afterthought to what is already written, then that means "Post Script" (Post= After, Script=writing; derived from word Scriptum, Latin), then, if there is an additional thought, P.P.S. is used, meaning Post, Post-Script, and so on.
http://www.answerbag.com/q_view/96556
Example:
Dear JohnP.S. Post-Scripts sometimes are used to convey a thought that is completely unassociated with the topic in the main writing. (Post-script; Think: After script)
P.P.S. Sometimes they are used to list ideas completely unrelated. (Post, post-script; Think: After, post-sript)
P.P.P.S. These could continue forever as long as you add another "P". (This one is Post, post, post-Script; Think: After, post, post-script.)
P.P.P.P.S. (In other words, Post- post, post, post-script; Think: After, post, post, post-script.)