From CA face book : Digital customers can begin preloading via Steam from 10:00 PDT / 18:00 BST today (28th Aug) in order to be as ready as possible to begin their conquest of the ancient world on launch day, with only the day one content patch including Pontus, an additional playable faction, needing to be downloaded once the game is live.
I expect amazon to complete the transaction in a few days before release. There will be adequate time to preload if you have a fair internet speed. You can get your steam code from the amazon page where your digital downloads are. At this time they are not released yet.
Five dollar credit download applies to mp3s, audio books, maybe even games from http://amazon.com.
Rome 2 will undoubtedly have a ton of dlc in the coming year, like previous titles such as Shogun 2. Hopefully there will also, be a blood pack. I expect further factions, more unit rosters for each faction and perhaps era campains such as one during the greek times.
It's available for preloading. You should have received an email saying your credit card was charged, and in your video games on your amazon account you have 2 codes (one for the game and one for the culture pack) that you have to put on Steam. Preloading can start once you've done that.
Just a heads up, As of 8/29/13 I have not been able to preload this. Or for that matter get a valid key. When I go to my game library there is no key shown below the title. Clicking to download gives me a small file but it fails to launch a working .exe.
I messaged support inquiring about a key and they sent me one. But this key was invalid within steam when redeemed.
So it may be awhile before this is resolved... hopefully before release day.