Can I print the return address on the back flap of the envelope?

The two passes

1. Print fronts. Click Download front PDF and print it as usual.
2. Reload for the back flap. Flip the printed envelopes over so the flap side will print. Keep the same top-edge direction and tray position as the fronts; if your printer setup is saved, Penmark spells out exactly how ("Flip the printed envelopes over. Keep the top edge pointing left and keep them centered between the guides.").
3. Print the flaps. Download flap PDF unlocks once the fronts are printed. Print it with the same paper size.

What prints on the flap

The return address, centered in a band near the top edge of the flap, in the style you chose; an optional heading line above the name ("The Future Mr. & Mrs.", for example); and, if you picked one, an ornament centered beneath the text. The two divider ornaments (Accent line and Flourish) print on the flap only. If a size's flap is too short for the ornament, Penmark says so in the preview and prints the text alone.

Why Penmark prints the two sides as separate passes

Most printers' duplex paths are not designed for envelopes, and an envelope's two faces are different shapes. Printing each side as its own pass, with the stack turned over between them, is the dependable approach.