From Official Pix:

Pre-order here through Saturday afternoon, April 16.

We are happy to announce that Gen Con has provided space for customers who pre-register for Celebration III autograph and photo coupons may now pick their coupons up Wednesday afternoon from 2:00 PM - 6:00 PM in the show registration area. This opportunity to get your coupons a day early will allow pre-order customers to proceed directly to the actors' signing tables as soon as the autograph hall opens Thursday afternoon or allow you to enjoy other aspects of the convention knowing your coupons are safe in hand.

As an added bonus, pre-order customers will receive the lowest numbered wristbands that will be utilized for the ten actors who we expect to have longer lines and who will have their own queues. (Temuera Morrison, Ray Park, Billy Dee Williams, Dave Prowse, Matthew Wood, Peter Mayhew, Michael Kingma, Jeremy Bulloch, Leeanna Walsman and Kenny Baker).

If you can't get by on Wednesday, you can still enter the Autograph Hall coupon Fast Path line with your PayPal coupon purchase confirmation any time during the show.

See the full autograph program details here.