It is the way a wigger (wiggah) /chigger (chiggah), a person who lacks racial confidence/pride in his own race and depends on identity upon trying to be something he's not (like being black), says "Peace" in order to seem black.
It is a way to say "I'm out" of a conversation when you are chatting online. Another way of saying "Peace" or "Duece", but being chill about it. And, chill / laid back now a days seem to be reflected by the word "Payce".
This has nothing to do with someone trying to impersonate a Black person, its the attitude / emotion being replicated.
Jeremy: "Say I got a lot of work to do. I'm out"
Jeremy: "Payce"
Kevin: "Later"