To sum up what the others have said, and from my own experience, is this:
If at all possible, avoid using a courier to send your items. Try and use USPS (United States Postal Service) for shipping, which in turn will have Canada Post deliver it for you once it crosses the border.
USPS usually includes and brokerage fees into the shipping, where as couriers seem to make a fortune off brokerage, customs and duty fees.
If you're not sure, its usually worth an email to the seller asking / requesting shipping via USPS on their end.