The idea behind this was to use the DL Window Envelope as Arkham Asylum, with all the bad guys inside. When you open it up and release them all there is a card with Batman saying 'Curses, you've let the scum of Gotham City loose!'.
I just thought it would be a little more interactive than just sending an envelope that had been illustrated to the site. Just hope they like it enough to include it in the next book.
Incidentally, the released 'bad guys' are crackback stickers!