A printer system is configured as a kit for retrofitting the input consoles of existing letter sorting machines of the type used by the U.S. Postal Service. Such machines require that an operator, stationed at a console, enter sorting information for each mail piece, by way of a manually actuated keyboard. The present invention expands the processing capabilities of the machines by imprinting machine-readable, coded information on the respective faces of the mail pieces simultaneously with the sorting thereof. At the same time, the printing function does not impair or modify the usual operator-controlled console when mail is processed which does not require such coding.