Auto Paste is cool. Turn Auto Paste on and the next time you start FastEnvelope the current item in the clipboard will automatically be pasted to your envelope.
Example: Type a letter in your favorite word processor (Have I mentioned that we will be coming out with our own word processor in the next few months?) and select the addressee's name. Copy it to the clipboard. Now open FastEnvelope. Bingo! All you have to do is type command-P to print.