It is very unlikely that your package will be inspected when you ship within Canada because it will not go through customs. If the package is obviously suspicious or if it is damaged and the content can be seen, then post officials they might look into it.
What I suggest for your C36 mags is to dissassemble them before shipping them therefore making it obvious that they are cheap plastic mock bullets and not real steel inert rounds.
If all mail within Canada was monitored, we would not have the luxury of calling this place a democracy
I hope this helps