If you are from a visa exempt country and have COPR, you actually do not need eTA but some airlines are not aware of this and might create problems. I have read in this forum people recommending to get eTA online which does not cost much.
When a person checks in for their flight, the system gives a "Board" or "No board" response. When the COPR is issued, an electronic authorization is also issued in that system to give a "Board" response. It is not an ETA but works the exact same way, so for simplicity's sake, I call this an eTA.