Not a problem. Just use those with a feeder parallel to the desk that pulls the envelope or label page throuh... rather than those that feed downward where the paper and envelopes are perpendicular to the desk. We have HP lasers widely distributed just for this purpose. We have great luck with older HP's as well such as the LJ 4L, LJ4M, and so on. I can post a number of HP brands to avoid... such as the multifunction printers, as well as the 1100, 1200, series.
Also fine are the Konica Minoltas, Samsung, and Fujitsu...
You will always have a jammed envelope now and then, but the problems of overheating have never been an issue.
If the printing is high volume, do a search at the HP website in printer support for the "Print to Mail" accessories... If they have a decent budget, the printers that use these accessories are the ones to get.