Largely agree with observation by
keesio who posted while I was writing this post.
CDNPR2014 said:
A Megabus employee is an ordinaory us citizen whose job is to drive a bus, and does not have authority to make a decision about what CBSA will and will not accept at the border. i'm sure yes, they have their "checks" that need to happen, but they are not the decision maker in the end. even if they tell you it's ok, it's up to the CBSA officer at the border, and they aren't going to care a random bus driver from the US told you it's ok. you are better off calling the CBSA office at the border you intend to cross at and confirm with THEM what is needed.
It is not about what CBSA requires at the border.
It is about a commercial carrier's obligation to not transport persons to Canada who are not carrying specified Travel Documents, and of course the fairly severe liability Canada imposes on the carrier if it fails to comply with the requirements. These requirements are more strict, relative to PRs, than what CBSA can impose at the border.
So the bus employee would be checking for the Travel Documents the carrier is required, by law, to check before allowing passengers to board a conveyance headed to Canada.
That said, services like shuttles can be casual or even lax. Been awhile since I used a shuttle service across the border, but that was going the other way and the Americans are almost always more anal about this stuff.
At the border itself, passport plus CoPR gets a PR into Canada. Does not necessarily get a pass on questions about compliance with the PR Residency Obligation, but again the passport plus CoPR should easily suffice once the OP gets to the border.
It is getting to the border which can be a bit of an issue.