Nikki, are you mailing them from the US? If so, it's US postage not Canadian... ask at your post office. Last I knew, the US has no special rate for Canada, just international, but does have a cheaper rate for postcards.
My Mom in Schenectady has had occasional problems with trying to mail to me in Canada from her home. She usually just puts anything she wants mailed in her home mailbox and the letter carrier is supposed to pick it up when (s)he delivers the mail. But sometimes they won't pick up anything with an international address and tell her she has to go to the post office to mail it! Weird. So you might as well check with the post office and mail from there.