ShareCon has a special envelope printing utility built in.
You must adjust the address placement (just once) to suit your printer. First and foremost, do your testing on PLAIN PAPER, not on envelopes! Look at your printer, and note how an envelope travels through it. The envelope is shown in light grey on the setup form. You can move the envelope to three different locations by clicking the L, C, and R buttons (left, center, right), matching your printer. Next, move the "to" and return addresses (indicated by red and blue rectangles) where you want them on the envelope outline. Once you think you have it right on plain paper, try it with an envelope.