(a)  Penalty mail is restricted to articles not in excess of the weight and size prescribed for that class of mail receiving high priority in handling and delivery, except⁠—

(1)  stamped paper and supplies sold or used by the Postal Service; and

(2)  books and documents published or circulated by order of Congress when mailed by the Superintendent of Documents.

(b)  A penalty mail article which is⁠—

(1)  over 4 pounds in weight;

(2)  not in excess of the weight and size prescribed for mail matter; and

(3)  otherwise mailable;

is mailable at rates for that class of mail entitled to the lowest priority in handling and delivery, even though it may include written matter and may be sealed.