Did you try this between 10pm and 4am, or did you try it outside those hours? It will say unavailable when you try to do this outside of the 10pm to 4am window. As the instructions at
http://store.apple.com/us/browse/home/shop_ipad/family/ipad say:
This is why I say "order online for pickup the next day".
For same-day, just pick up the phone and call.