I already got my PR card but thought of a quick post here to share with anyone who needs to travel outside Canada urgently and are still waiting for their PR cards...did not see anyone share details on how exactly it was done. I'm sure someone in the future who accesses these posts from Google will have similar doubts about the process.
I am not from a passport-exempt country, and had to fly urgently out of Canada before I received my PR card (had already bought an RT ticket in/out of Toronto). As I already had a US visa, I had to change my return leg to land in Buffalo, NY. I then rented a car and drove through the Niagara Falls border...it was a very smooth process! Didn't even have to step out of the car. Was a drive-through kiosk...the officer just asked for the purpose of my visit, and I said I'm a PR and showed her my CoPR letter. She just looked at it, welcomed me back home and that was it...it maybe took 2-3 minutes at the kiosk.
So while it's an expensive option (to cancel/re-route the air ticket, rent a car from Buffalo, and then drop it off in Toronto), it is also a very smooth alternative...so no need to be worried about returning back to Canada (as I was) without your PR card.