... I haven't got any experience with 2010 (only 2009 and 2003), but... In 2009, as part of your wave documents, you can print various (pickticket/packing) reports as well as carton (shipping and/or contents) labels. You could look at some of these and do it in such a way you could use them for this purpose.
Alternatively, you could look at creating the returns label with your outbound documentation. Or, you could go, for instance, with a peel back label on your despatch note. This would actually save you printing a different lable/document (but the cost of your basic invoice will probably be slightly higher). It just depends what fits best with your operational flow.
Hope this helps.
Chris