As a person who makes money developing Android apps, I have to tell you that you probably want . . . a mobile website.
A mobile website is probably 5x cheaper. Imagine hiring a developer for every platform instead of expanding the web development team you probably have.
HTML5 is said to be the way of the future.
Amazon has an Android App AND a mobile website. They don't look very different in style. I like the mobile site somewhat better.
For $50-100 bucks, you can find several websites that will create an app that just launches your mobile website. Then you can say you have an app.
There are a few exceptions:
If the store is for things people will use on the phone: ringtones, mp3s.
If you can make it a compelling vertically integrated tool.
Example: Shop at Ted's Model Rocket Supplies isn't all that compelling as an app.
But a Model Rocket Designer App that lets you design and simulate your own model rocket using a 3D interface and then push a button to order all the supplies you need in one click - that's more compelling.