I love the ease and rates of shipping on Shopify. Its much faster then having to use a third party app. Its wonderful that you can now print packing slips right where you create the labels. I would love to see a feature where you can print both the label and the packing slip right on the same 8.5x11. Most third party shipping apps let you do this. It would increase productivity/workflow. For example: Online Labels OL830IF would be nice to see in Shopify's shipping service.
To re-highlight, packing slips are made for shippers to be aware of the order and consignment details. They can see what they have ordered and what has been done. A shipping label is a document that is affixed to the packaging and helps the carrier identify and track shipments.
To print both the label and the packing slip right on the same 8.5x11 you have to switch to other apps like Multi Carrier Shipping Label App. The app allows you to format the size according to your needs.