I believe that Costumebase is based in Singapore, and not in the USA.
From what I've heard from the Star Wars cosplayers, it's a 50-50 thing with Costumebase. Their products aren't the worse, but they aren't the best either.
You could try to order here from E-Bay in the link below, and order expedited shipping. (It won't be cheap for expedited shipping.) I'm not sure if it will make it to the USA in time, because 15 days may not be enough time for international shipping from Singapore, and sometimes customs would hold it up. (Try to pay through Paypal, so you if they don't send you the product, you could dispute it within the normal time limit that is set by PayPal.)
The company in the link is from Singapore.
I would also suggest contacting the Rebel Legion, and your local Rebel Legion Garrison or Outpost in your area. They should be able to point you to the right direction.
Perhaps if time has ran out, you could consider going without the helmet? Princess Leia went helmet less for some time in "The Return of the Jedi".
Good luck, and May the Force be with you!