An interesting extension to the snail mail services would be to snap/upload a photo, use geolocation to find the user's current location and automatically send postcards "from" anywhere in the world.

I always send my postcards the last day and then hunt for the stamps and post box/office where I can send them from. I'd really find this service useful :) Although, arguably a big part of receiving a postcard is the stamp which you wouldn't have here.