As others have noted there are options - here's three more:
rent a car at the pier, use it all day then drop it at the airport when you're leaving;
arrange a tour that will pick you up at the pier, sightsee all day then drop you back at the airport (or just arrange a car and driver, same thing);
Ship it to yourself - the post office, Fedex/UPS/delivery companies. May be more expensive/complicated since you're shipping internationally (US to Canada), but we've done this a couple of times at the end of a trip with things we've bought along the way.
Not likely, but also see if the airlines has an office in the city where you could check in. I don't think any airlines do that in the US, but I've seen that in Asia and in Europe and it simplifies things.