Why does Postbox purchased from the Postbox Store appear to be “Installed” within the Mac App Store?
The Mac App Store may show software bought from the Postbox Store as “Installed,” even though they’re different products. If you wish to purchase Postbox from the Mac App Store, simply drag Postbox from your Applications Folder to the Trash, empty your Trash, restart your computer, and the Purchase button will then be re-enabled. Your mail settings and mail databases will not be affected.