The waiting for the answer is killing. I'm expecting the answer later today or tomorrow morning.
IMO this is how it should work. You would sign into your account and see a button that says "Click here to JOD to iPhone 6S or 6S+". You click that button and it then shows you the color options as well as GB size just like on the Apple site. Once you pick size and color it tells you ship dates. This way you can see if you'll get it launch or after. Could effect size and color you choose. If you previously paid the down payment you would just put in shipping address, otherwise you pay the down payment, put in shipping address and you're done. It would be straight forward and then you just wait.