I don't know anything about Canada's mail service, but for US mail you should report theft (including mail delivered with contents missing) to the Postal Inspectors.

https://postalinspectors.uspis.gov/c...complaint.aspx

The USPS is a huge organization employing a huge number of people. Unfortunately not all of them are honest. If incidences of theft are reported to the Postal Inspectors, they can look for patterns that can help them track down the criminals.