Address the envelopes before stuffing them. Then insert your invitation into the envelope with the text facing the flap side.
To create a reply card, buy 50 postcard-size flat cards in a coordinating color (about $7 for 50). Print your RSVP information on one side and your return address on the other.
Slip your reply card and any additional wedding information (such as a map and directions) behind the invitation.
Take completed invites to your post office and have them weighed to ensure correct postage. Visit www.usps.gov to look up zip codes and rates.