Why not custom postcards that add a vintage / rustic flare to your wedding invitations. They also make it really easy! All you have to do is put a stamp on them and then your guests simply 'Reply' and pop their poscard in the mail and it will come right back to you!
These postcards would feature a photo of you as well as your date and initials, with your address on the back and a space for your guest to fill in their reply!
Take a look at the listing in my Etsy shop to see how you can order your very own custom RSVP postcards today!
Front of Postcard: