I seriously doubt it was Canada Post that opened your mail, big big no no for them to do it, if an envelope/package came open while in transit to your place they would have sealed it with tape. In Canada they just don't do that like might happen in 3rd world countries. My first thought was your landlord, are you renting a suite in their house? Check with CP and see if they can hold your mail at the post office and you can pick it up whenever you want or get a PO box there.
My brother had an experience where his landlord thought they had the right to open their tenants mail because it came to his address. Maybe ask if he opened you mail by accident. If he did best to try and get it held at the PO, you never know when he might make another mistake.
It is against the law to open another persons mail in Canada, inform your landlord if he isn't aware of that.