It seems like a poor policy to just trust that other people are dropping the guns off in a safe condition.

Guns shipped from retailer/manufacturers have a smaller risk of being improperly stored or loaded when shipped.

These concerns don't track with literal decades of experiences with individuals shipping firearms to each other and for factory warranty.

There is still a risk of a Canada Post employee realizing these boxes contain restricted firearms and arranging them to be stolen/stealing them themselves.

This is a much, much bigger concern. Canada Posts chain of custody is not the greatest on good days with mundane mail.