Returning to the "Shipping Agent Card" you need to select the label type(s) you wish to use from the Labels FastTab and for each Label you are able to define further information for the Label as detailed below. 

Select Label Setup from the menu to open the "Shipping Agent Label" screen for the selected Label.

Note: you should only complete the printer setup fields if you only have one printer for printing this shipping agent’s labels. If you have more than one printer for the Shipping Agent you should setup Label Printer Selections so that you can control printing to multiple printers

Default Printer In correlation with the Print Handler defined in "Shipping Agent Integration Setup", use this attribute to select the printer where Labels will be printed
Printer Tray If your printer has multiple Printer Trays, you can select the relevant Tray using this setting