I would feel more comfortable using a service like this if there were some sort of "About" page, describing who's running the show and why we should trust you. (Maybe there is, but it's not easily found?)

Also: More information about process.

For instance, where will these letters be postmarked from? For many developers, that's kind of important. Some might want the cache of a large city. Others may simply be concerned about the time it will take to reach the recipient.