The best way to print the envelopes is to use a printer dedicated specifically to envelopes. We have tried printing envelopes on big commercial printers and also high speed laser printers.
The problem with most of those printers is that the heat from the larger, faster printers heats up the glue on the envelopes and glues the envelopes shut.
Chris recommends the Epson XP 410 for printing your envelopes.
Another option that we have used is the HP DeskJet 5440 Photo Printer. It will print about 500 envelopes in an hour and even more importantly it does NOT heat up the glue and cause it to bind together.
If you are interested in Printing Companies, check out this article: What Printing Companies Have You Used?