You need to be clearer about what exactly you want to 'automate'. If the merge document is already populated and data attached, then you merely have to click the button on the mailings tab to complete the merge to a new document or the printer.
If you want to print a range of labels from an Excel data source, then you could use
http://www.gmayor.com/Envelope_Label_Add_In.htm. It is not mail merge, but the end result is the same ... except you can select the start label position.