Hi Dalaimamma,
First of all, excellent choice of name. But for your question, in order to choose a different carrier, all you need to do is create separate zones for Canada and USA in your Shopify settings. In Shopify > Shipping in your profile(s), you can create separate zones for each country, and then, when adding rates, it will only show the rates included in said zone to the customers, ie. if you have a USA zone with printful and CAN zone with another carrier, a CAN customer would not be able to see the printful rates.