Can I convert a purchase from the Postbox Store to a copy managed by the Mac App Store?
There’s no way to convert a Postbox Store purchase to a Mac App Store purchase without repurchasing Postbox on the Mac App Store. 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.