It is illegal for a non-American citizen to purchase ammunition in the US. It is illegal for a vendor in the US to aell an Alien ammunition. That being said, under Canadian law you are allowed to bring in 5000 rounds per day. You will be required to pay Canadian taxes on your purchase. I do buy shotgun shells in the US by going to a large sporting goods store where the ammow is kept on a serve yourself counter shelf. I put what I want in a cart -- take it to the checkout and pay in Amarican funds (no cards). I am very careful to obey all speed limits etc.. If you are stopped by the American police for some reason and are found to have ammow in your possession, you will go to jail
Cheers, John