I agree with the comment that the email hint is totally unnecessary, and the ambiguous 3 day expiry window is too confusing in an world where post might take 1 - 2 days to arrive. Additionally the postcard aspect means people at every point along the delivery chain can read the back and front, why not just send a letter?

As a side note, I really like the way the author of this blog has constructed his "Contact Me" bit at the bottom, it's very intuitive, clever, and uses URI schemes where appropriate, nice job!