This looks awesome; I've wanted a service for sending mail for a while. Cards seem like a nice place to start; hopefully they'll expand to a few other form factors in the future.
Personally, I'd love to see this for a couple of data-related use cases: mailing either USB disks or CDs based on an uploaded image.
Back in 2001, a sufficiently large company could upload a .iso to an FTP server, and Cinram (or other disc burning companies) would burn and ship on demand. I'm pretty sure they had a web API that was integrated into the ordering backend of my employer at the time.
Personally, I'd love to see this for a couple of data-related use cases: mailing either USB disks or CDs based on an uploaded image.