Can't buy zip addon. Says it's already installed.

I would really like to purchase the zip addon, so I went to the page where the premium addons are available. When I scrolled down to buy the zip addon, the table there shows the price, but next to it is the word “Installed” rather than “Buy Now” http://screencast.com/t/COknEI2B8X.

Even it I did have it installed (which I don’t) how would it even know that enough to say “Installed.” How can I buy it?

Thanks.

http://wordpress.org/plugins/wordpress-backup-to-dropbox/