What is the test for proving that a letter was sent by ordinary mail?

I recognize the difficulty in proving that a letter was sent by ordinary mail, particularly where one is dealing with an office in which a large volume of mail is sent out every day. The best that one can usually do is to establish that the usual procedures for dealing with mail were followed (as, for example, in Adele Schafer v. The Queen, APP-478-96-GST). I also recognize that mail sometimes get lost and that occasionally documents become lost in the department itself. However the chance that two separate letters would go missing is very low.