RSVP postcard at the end of a seal and send invite. Also addresses the "Plus One" issue.
Custom Retro Silhouette Wedding Invitation Suite with RSVP postcards and address labels. $1.60, via Etsy.
Fingerprint No. 2 Postcard Set // set of 8 letterpressed postcards // $12 // http://www.chendesign.com/shop/fingerprint-no-2-postcard-set/
This is postcard design, but would make interesting wedding invites.
Agents: Stop Sending Boring Postcards... Send Ones Like THESE! (10 examples)
Just one in six of us still send postcards. Here are some examples of what we're missing.
Real Estate Blogging
Dervish, postcard from early 1900s
Art - Past Photography
An example of the postcards in the Curt Teich archives.