Envelopes that you run through your laser printer look beautiful when they first come out. They look like they've travelled through a war zone, though, by the time the letter reaches its final destination. Here's why: lasers use toner which is dropped onto the paper in small raised piles and then run through a fuser which heats the "pile" at about 400ø Fahrenheit, melting the toner to form raised letters. Feel an envelope or lasered sheet of paper and you'll see what I mean. Now scrape that image with your fingernail. Some of the "fused" toner scrapes off.
Place two lasered envelopes together and rub them back and forth. Notice slightly cracked letters? Now imagine your envelope running through the USPS automatic gizmos and scraping against the equipment and other envelopes along the path. I've sent some letters out that lost 50% of the image by the time they were received. Makes a lousy impression.
Solution: buy an inexpensive daisy wheel printer (costs about $200-$300). The daisy wheel imbeds the ink into the paper fibers and makes a much stronger and harder-to-damage impression.
Benefit #1: The laser printer's heat tends to seal some envelopes as they run through the machine. Daisy wheel printers run at about room temperature and don't accidentally seal the envelopes.
Benefit #2: Lasers will print envelopes at 4-8 pages/minute, less time out for jams on every fifth envelope. Daisy wheels can print 8-10 per minute and rarely ever jam (my Primages Daisy Wheel averages 12-15 per minute).
Benefit #3: Cost. Laser envelopes eat up toner and wear down the cartridge's drum (because the envelope is narrower you tend to get a toner build-up along the edges, which means you may get a "streaking" toner image near the edges when you switch to regular size paper). Envelopes cost about 3 each to laser (those toner cartridges are expensive). The most expensive daisy wheel will cost less than 1 per envelope (one $10.00 ribbon is good for at least 3,000 envelopes on most printers)