If you get this error on submitting your app to the Mac App Store:
Invalid Signature – This error occurs when you have signed your app’s installer incorrectly. There are two certs required for this process: the “3rd Party Mac Developer Application” cert and the “3rd Party Mac Developer Installer” cert. When signing your package, you need to ensure that you are using the Installer cert to sign your package. Ensure that you are specifying this cert when submitting your app via the Xcode Organizer or when running productbuild from the command line.
Then check out this newer post which contains the updated details!