I've never been to these things, so I don't know how it works - you have to mail in something if you want a specific thing autographed? Then what happens? It's not signed in person?
Usually you either take something with you and he signs it for you or you choose from the bunch of photos they have available.... or if you have a photo taken you can go and collect that and get that signed.
They always do signings after photos for that reason.