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.
When setting your retail prices or manually editing them, keep in mind that customs agencies often calculate duties/fees based on the declared retail price.
If you collect VAT in your store, note that Printful doesn't currently offer a way to display paid VAT or a VAT ID on shipping labels or packing slips.
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 tracking packages so that they're less likely to get lost in the mail.