You must start at the TOP.
You SERVE the POSTMASTER GENERAL in Washington, DC
---AND---you must also serve the ATTORNEY GENERAL OF THE
UNITED STATES in Washington, DC as you're SUING a FEDERAL
Send them CERTIFIED . . . DON'T FEDEX OR UPS them.
THIS ANSWER IS PURELY FOR ACADEMIC DISCUSSION ONLY AND DOES NOT CONSTITUTE ANY TYPE OF LEGAL ADVICE OR LEGAL REPRESENTATION.