Yes, Shopify does allow you to set up a default packaging.I also agree that in many cases where the cost is calculated based on weight and product size, the shipping quotes do not come out correct. For such cases, you need to use an app that has Canada Post and secondly that uses dimensions for calculation. Using the Multi Carrier Shipping label app, this can be achieved. Basically, with this app, you can define the multiple packages and also assign individual product dimensions. Once that is done, the app uses a stack packing methodology to pack items into boxes and give you accurate shipping quotes based on weight and dimensions.