Following on from the Packaging Tab, is the Labels Tab again the defaults in this screen come from the installation of the Shipping Agent App and in turn the documentation sent form the Shipping Agent Service.
Like packaging you will be required to go in and edit the label cards. This will include changes to label height and width. There is also a field called Default printer, this will need to be populated for the label you are using as part of this Shipping Agent.
This bit of setup here links back to the label code you filled in on the General FastTab.