Before Canadians ship to the US, they will need to register with the US FDA and obtain a Food Facility Registration Number. They then need to give prior notice to the FDA via their online system before shipping an order.
Of course, we encourage cross-border commerce! Canadian buyers can buy from US sellers (and vice versa). However, if selling across the border sounds like more work than it's worth, just turn off "Allow International Shipping" on your account page (Note: this option only appears to registered sellers).