Did you try contacting customer service directly? As a Canadian, I'm all too familiar with these automated check systems validating addresses, and they can be a real pain in the butt to maneuver around even when you add a US billing address to your credit card for just such purposes.

Customer service is very responsive, and since you do technically have a card with a US billing and shipping address, maybe they can massage their system via a phone or email order. If you didn't have such a card, I'd say that yes, a 3rd party is probably your best option. But since Entourage customer service is outstandingly excellent, they might actually be able to help you in this case if you contact them.