Gotcha, You're right. I should wait for the actual work permit document to arrive before applying. I might as well wait a week or two instead of taking a risk.I applied online using the PGWP Copy that arrived in the mail. I am told you can do it without it as well. Knowing IRCC, I did not want to chance it.
That said, have you received any other electronic document after WP-EXT for PGWP or is that the last message on your approved work permit application?