Mail of any class, including mail indicating Special Services (except Priority Mail Express service, Registered Mail service, and nonmachinable First-Class Mail service), that is received at either the office of mailing or office of address without enough postage. Such mail is marked to show the total (rounded off) deficiency of postage and fees. Individual such pieces (or quantities fewer than 10) are delivered to the addressee on payment of the charges marked on the mail. For mailings of 10 or more pieces, the mailer is notified so that the postage charges may be adjusted before dispatch.
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