This tip will let you print on Hadoe's card envelopes.
You can try this at the HTML testbed or a blank page. This works assuming the rounded flap is the top of the envelope. This is set up for HP, you'll have to adjust the spacer size for Canon. Print the envelope first, don't cut it out. After bringing up the results from the page you chose to type this on, put the printed envelope back into the printer printed side up for HP, down for Canon, rounded flap to the inside of the printer. That will print in the center of the envelope on the clean side of the printed envelope.