Not all CP locations are actually CP. In my city, there is only 1 CP location at at least 10 contract locations, with contract workers.
You are not allowed to indicate "gun" or "firearm" on the packaging, nor are you supposed to tell the postal outlet employee. But where I'm from, they get thousands of guns shipped here every year and CP is the only legal way to ship them person to person as an individual boxed item (moving companies notwithstanding). They very often know what's in the box, just by the shape (it's happened to me).
Read here, as this applies to replicas also:
And CP or it's contractors have every right and are sometimes obliged by law to scan your package, if it's going by air (and you may not be able to chose a non-air service depending on location).