Abstract: An add-on to a standard multi-tray duplex printer (7) directs, from the printer to a holding and creasing station (14, 15, 16), a standard envelope-size sheet (1) (e.g. A3+) printed with at least a sealer toner frame (4) on the inside and a printed address on the outside. Content sheets (5) (e.g. A4) are then printed and stacked at a collection station and the complete stack (5b) is then folded into the envelope sheet (1) as rollers (15, 16) press and seal the toner borders together, delivering a sealed and addressed finished envelope packet (1b) in a single line from a standard printer using standard paper sizes in standard magazines. The invention includes the add-on unit, the printer with add-on and the enveloping method.