Shipments outside of the USA may incur customs fees depending on the destination country. The fee may vary depending on your order value, country limits, and other factors based on the product itself. Your end customer is responsible for these fees. Any fees are paid to appropriate customs agency by the end customer.
With Printful live shipping rates, orders to Canada generate an Int'l Economy Duties paid with tracking option (4-8 business days after fulfillment). This method can be useful for avoiding lost in transit packages.