It seems that letters to offices at the House of Commons are postage free!!
Based in Montreal/Quebec
Justin Trudeau LiB-PM
Office of the Prime Minister
80 Wellington Street
David Lametti (LaSalle–Émard–Verdun) Minister of Justice and Attorney General Contact David Lametti
Steven Guilbeault MP (Laurier–Saint-Marie, Quebec), Minister of Canadian Heritage Contact Stephen Guilbeault