Amazon currently don’t have a way for us to sell Campaign 2, the only way you can do this at the moment is to purchase the game through Android Market.
I had originally purchased Sentinel 3 from the Amazon Appstore and I ended up having to re-buy Sentinel 3 from the Android Market and then turn around and purchase the second campaign, as well.
On the one hand, it’s an awesome game and I really enjoy it. But on the other that didn’t exactly sit well with me either, since I already *owned* the game.
Amazon took a long time to release their In-app purchasing system to developers. It is now released, and we are working on implementing it and bringing all the latest features to the Amazon version. We don’t have a firm release date yet, but rest assured it is on it’s way