I don't think the US site ships to Canada so those close to the border will often ship it to a mail place designed for that purpose. Duty/tax would be at the discretion of the border guard depending on how much you spent.
I know, right?? I don't get it because we have maternity in the stores so why not online? However we do have plus sizes online now so maybe in time? Canadian preggos unite...we must ask ON to put maternity online!! LOL