How annoying for you. I would definitely take it back and point it out to them and see what they say. Hopefully they agree to exchange the outfit if not I would get a bit more forceful (while still being polite) and tell them you want a new outfit as it isnt good enough providing you one with pen on it especially considering you paid $180 for it. If they dont come to the party I would probably ring Consumer and Business Affairs and find out exactly what my rights are.

Good luck
Jem