I ordered my invites from Vistaprint, 100 invites, reception cards, and rsvp cards for $130 including tax & shipping... i'm super happy with them, they look great, and I had ordered the save the dates from there so everything matches. WIN!
The only concern I was having was envelopes. Vistaprint sends a thin, white envelope. No inner envelope. And, I ordered the 6x9 invite, so I was worried I wouldn't find anything to class them up a bit and couldn't find anything for the 6x9 invitations on any of the forums - I guess everyone just skipped the inner envelope.
I wanted to write just to let people who might have the same question know that there is GREAT NEWS!!! The "A9" size envelopes (i'm getting mine from envelopemall.com) fit perfectly as an inner envelope. The invite fits nicely, and the A9 fits great inside the thin white one that comes with the invites! Use an A9 as your inner, and you'll be set to go!