(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.