A couple of things from my experience:
1) Most recommendations are online, and won't need stamps or envelopes. Of course, ask your recommenders if they prefer to send in a hard copy, if that's an option. They probably won't prefer to, but it is good to offer that if it's available. If you do need to have hard copies, I would send them a package with the required forms and stamped, addressed envelopes for the appropriate schools.
2) Give them lots of notice! I sent out a request several months ahead of time, and then gave them the specific instructions regarding which schools they would be hearing from (because my list changed between the first request and the time I applied, and because I had different recommenders for different program) 2-3 months before application deadlines.
Good luck!