I am also trying to figure out the PID number. I believe it is an identification number that is given to us on the my unc page once it has been updated. If you find out please post what it is!
Ah so exciting. My letter was in a 5x7 envelope and my myunc has not been updated yet- I saw on past forums that it is not updated for a few days after the letter is received. Good luck!!! :)