QC is the province of Quebec. Quebec is officially Francophone, and as such would never use QC to designate Quebec City: that's only in English. For envelopes, the accepted block form in Canada is:
Name Street or Post office box# City, Province [two-letter designation] postal code
Postal Code is L#L #L# where L=letter, # = number eg: my postal code is V9X 3E1 If there is a lack of space, you can put the postal code on the same line as the City, Province, so long as there is an identifiable space between. Canada Post prefers that postal code be on a separate line to allow the machines to read it more easily. Postal code is more important than Province if you want your mail to get to the right place: [V = British Columbia.] You can look up Canadian Postal codes here:
formatting link
The accepted provincial Postal abbreviations are: Alberta AB British Columbia BC Manitoba MB New Brunswick NB Newfoundland and Labrador NL Northwest Territories NT Nova Scotia NS Nunavut NU Ontario ON Prince Edward Island PE Quebec QC Saskatchewan SK Yukon YT
There....more than you ever wanted to know about Canada's mail system;