1. A customizable user interface for a mailing apparatus, comprising: a) a control program, including instructions for execution of all functions of the mailing apparatus, and including instructions for painting a screen of the customizable user interface in response to external input data specifying the screen, and including functions required to check and respond to an improper response to the customizable user interface, responsive to the external input data; and b) the external input data, providing all specifications needed to distinguish one screen from another, and specifications needed to distinguish an action by the mailing apparatus in connection with use of a screen by a user, or in connection with an event to be communicated by the customizable user interface to the user via a particular screen.