If you get registered with the post office you can get a number called an MID or mailer ID.
It's something that you should be able to have in encoded in the tick marks on first class letters and flats. It allows people to see where they are in the post office system without having first class tracking.
For those who ship a lot of letters and flats, allowing us to use our own MID's instead of stamps.com's would allow us to provide better service to our customers.