I'm Canadian and my father always tipped the mailman in cash.
I have no idea what the rules are for Federal employees, but assume that are the same for the Ontario Provincial Government employees, as I was one. No gifts can be accepted, cash or otherwise.
Annually, I tip the mailperson, the counter clerk and the Postmaster (all are females) in cash as I receive great service from all of them. They've never refused my gift. They know my name, my collecting interest (Canadian mint) and on request always hand stamp my outgoing mail to family and other collectors. I'm also given all of the retiring Canada Post
stamp and other philatelic posters when the new ones come out.
I also tip my barber (although she refers to herself as my stylist).