I think PPU is the better of the two.
+P is a good suggestion but some people will turn it into P+ and then we’ll be thinking they’re asking about Premium Plus. And yes PP is sometimes used to mean Premium Plus but is widely known everywhere for Priority Pass.
“Hi, I booked in E+ and 20 +P have cleared to put me in P+.. If the additional 20 +P don’t clear I’ll be stuck in P+ and need a lounge to use with my PP”.