When you click buy and follow the screens through it should show there will be no charge.
The better way to do this is to go to
Windows Phone | Cell Phones, Mobile Downloads, Mobile Apps, and More | Windows Phone 7 and log in, then go to your account information. It will show all downloaded apps, including purchases available for reinstall. Now I have a problem because it seems to only go back so far and then errors when I go to the page.
Also there is an app that will auto-install all of these items for you on the marketplace called reinstaller. However, it works off of the windowsphone.com pages and I receive an error in the app. Purhaps I have too many downloads, you may have better luck than I do.