Since the USPS/UPS/FedEx shipping gateways do not actually request an actual shipping label you would not be able to generate a shipping label for actually shipping the order. (think with the rate already billed to your UPS account and things like tracking enabled)
It is fairly easy however to use the Store order tags along with a special template and stylesheets to create packing slips, box labels and any other thing that you might want/need for your store admins to help with order processing.
I for example have built out front end admin templates for customers that allows me to export and print a batch of pull sheets, packing slips and basic shipping labels for new orders. I am also able to use the exported data to import it into their shipping management software.