Thank you for reaching out with this question! Shopify Shipping primarily encapsulates two main functionalities; the ability to buy shipping labels directly via the Shopify admin, and also the ability to offer live calculated shipping rates to your customers at checkout. For both cases, you can utilize them for Canadian customers but use other methods for those in the United States, if you desire. I'll go into more details below.
In terms of purchasing shipping labels, Shopify Shipping allows you to purchase shipping labels from selected carriers if you're in the United States (USPS, DHL Express, and UPS) or Canada (Canada Post). If you are in Canada and wanted to utilize this feature when shipping to Canadian customers, you can buy and print Canada Post shipping labels through the Orders section of the admin. If you were based in Canada and wanted to use a different fulfilment service outside of Shopify when shipping to customers in the United States, that would be absolutely fine - rather than buying a label directly through Shopify, you would simply buy the label elsewhere and ship the package, and then manually mark the order as fulfilled within Shopify. You can enter in a tracking number at this point, which will be included in the fulfilment notification sent to the customer. The same process would apply if you were in the United States, except you'd have the choice of using USPS, DHL Express and UPS when using Shopify Shipping.
Simply put, when you're on the Order page, you will choose "Create shipping label" when using Shopify Shipping, and "Mark as fulfilled" when not:
In terms of offering shipping rates to your customers, the principle is generally the same. When you create your shipping rates, you'll be doing so via the use of shipping zones, which allows you to set separate rates for different countries and regions. In the example you've given, you could therefore create a shipping zone for Canadian customers where you're using the live rates accessible to you via Shopify Shipping, and another zone for American customers where you can choose to offer different rates. Outside of the live rates Shopify offers, you can can set shipping rates based on weight or by price, or you can use third-party live rates offered by a different courier or app. I would point out that using these third-party live rates would likely require our additional carrier-calculated shipping feature, which comes as standard with the Advanced plan and can also be added onto all other plans for an additional $20 per month (or for free if you switch to an annual billing cycle). If this is something you'd like to do, feel free to contact our support team and we can walk you through how gain access to this feature.
In the example below from the shipping settings of my test store, I am using the Canada Post rates made available through Shopify Shipping for my Canadian customers, and for American customers I am offering flat, price-based rates:
I hope I have explained everything you were curious about regarding Shopify Shipping, but if you have any questions on the above please send me a reply and I'll be happy to answer any further queries that you have!
Victor | Shopify Social Care
You can definitely use Shopify Shipping for Canada by enabling it for Zone Canada inside the store. You can set up flat rates for the other zone(USA) or if you are looking to get rates from any of the carriers like Canada Post, uPS, USPS, FedEx, DHL using an app, you can try the Multi Carrier Shipping label app. You should not face any problem in using an app simultaneously with Shopify Shipping.