This site makes it easy to print any number of envelopes quickly and easily in your browser, all for free! It's far easier than normal mail merge. Just type or paste in a list of recipient addresses which you want to send letters to and hit "Print". Envelist will automatically generate a PDF of all the addresses and prompt you to print the envelopes.
It supports any size envelope, and there's no charge or downloads required. It just uses your own browser and printer.
If you have any other issues or questions, please email me, and I'll try to help however I can.