1 Labels are preset at 4" x 6". Go to admin and change size to 7" x 3"

2 Create a label using the USPS Letter option in the pre-select drop down menu for size.

3 click on the printer icon when the label has been created and you see the popup window with the label.

4 The "Paper Size" should be set at "Env10" and the "Scale" at "Fit to page". 

5 Simply insert an envelope into the printer as indicated in the printers manual (usually centered in the paper tray), and the postage will print directly onto the envelope. 

You can reprint labels over an over to try different settings. However, do not mail more than one of the labels, as this may be a Federal Offense.