Cool! I'm absolutely retarded when it comes to going to the mailbox to deposit mail-it usually sits on my desk for a few weeks first. And I never seem to have stamps. I think you should organize the card designs into categories, though, to make them easier to browse. This isn't a huge concern now but it will be as you continue to add to your design collection.
I did get an error page when I tried to connect with Facebook- Here's what it said:
"The browser has stopped trying to retrieve the requested item. The site is redirecting the request in a way that will never complete."
Also, you might want to try retitling this post: Ask HN: Review my Startup. I think you'll get a better response if its more clear that you're asking the community for input.