In 2011 at a hackathon I built rsvp.io (now points to a silly game of mine) to allow wedding couples to easily create a custom wedding invitation rsvp page. A unique code could be added to a paper invite, or the page used on its own. After the hackathon I spent another month or so refining the site. I never took it far enough, but might have made a little money had I tried. Some ideas that I never took forward were partnering with printing agencies for paper invites, and affiliate income for gift registry.
It did get used once however, for my wedding in 2013 :)
I can visualise this being used - you can have a cute-looking QR code (surrounded by a shape/design of your choice to not make it look "standard") to rsvp. You can even have stickers printed (instead of complete cards) that can be stuck on everybody's physical wedding card.
It did get used once however, for my wedding in 2013 :)