(a) This part applies to all materials that pass through a Federal mail center, including all incoming and outgoing materials. This includes:

(1) First Class Mail;

(2) Standard Mail;

(3) Periodicals;

(4) Package Services; and

(5) Express Mail.

(b) This part does not apply to shipments of parts or supplies from a material distribution center. A material distribution center is a warehouse that maintains and distributes an inventory of parts and supplies.