I got in this habit a few years ago when I was temporarily moved to another office on campus and the folks over there (wisely) re-used envelopes.
This is easy to do at work, but also at home if you haven't switched over to totally paperless billing yet, you can re-purpose the envelopes sent with your bills. Just keep a little stack of these and when you need an envelope but it doesn't have to be "fancy", use one of these old ones instead of a new one.
And of course, recycle your envelopes with the rest of your paper. Yes, even if they have the plastic "windows" in them. Win-win!
Pocket: 7, Planet: 13, Win-Win: 40