Return shipping label is something which is a necessity in today's eCommerce shipping. You can absolutely use return label feature within Shopify for Canada Post using the Multi Carrier Shipping label app. These are postage paid labels, so you will be automatically deducted from your account for any return label generated.
If you want to create a return label for Canada Post, you need to create a product (e.g. Return label), and assigned a dollar value to it, or set it to $0 if you are offering free return labels. Switch the customer address to the company address, and switch the company shipping address to the customer address. We don't let Shopify manage inventory, so read carefully the impact of doing this if you allow Shopify to manage inventory. Complete the sale for the return label, and generate the label. then reverse everything you done with the addresses, and hide the return label product. The question Is, is it worth it, just to save 21% on expedited parcels for domestic Canada Shipping. Probably depends on your return volume.
Since it can be done manually, why doesn't Shopify code domestic Canada Post return labels. I love Shopify, but it is the little idiosyncrasies that drive me crazy, like implementing a feature for the benefit US customers, and then either forgetting or take years to implement it for Canadian customers. You are a Canadian Company, please show some more love for your Canadian customers.