39 U.S. Code § 3207 - Limit of weight of penalty mail; postage on overweight matter
(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—
(b) A penalty mail article which is—
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.
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