You can ask the seller if he will ship by USPS. Keep in mind that a 16x20 " poster would probably be shipped in a tube and that adds to the cost.
His easiest solution is to make a private listing for you, using your name as the title, and adjusting the shipping for you. He may find Fixed Price cheaper but if he chooses Auction, it should be 3-day at most.
Let him know that if he prints the postage online, USPS First Class International Parcel includes Confirmation of Delivery to Canada.