Descartes ShipRush USPS includes support for Letter Post. Letter post prints on 4"x6" labels to a Zebra or laser printer.
First Class Letters and Flats are letter post and are not trackable. First Class packages include free-of-charge Delivery Confirmation.
If using #10 Envelopes, use a laser printer with an envelope feeder for the best results.
See Printer: Letter Post for details on configuring a printer for Letter Post.