If you are interested in seeing how Piaggio mounts their top case on the MP3 look here http://thekneeslider.com/archives/2006/11/20/piaggio-mp3-winter-accessories/#more-1254.
With a 48 liter top case on top of the underseat storage you could haul home a lot of beer. Maybe I will need the 400 cc model just to handle the extra weight.