7 C.F.R. § 355.21
(a) Any terrestrial plant which is to be imported by mail and which may be imported under 50 CFR part 17 or part 23 only if accompanied by documentation, shall be mailed to Plant Protection and Quarantine (at a port authorized for such purpose by the U.S. Department of the Interior in 50 CFR part 24 pursuant to section 9(f) of the Act (16 U.S.C. 1538 (f))); and shall be accompanied by a separate sheet of paper within the package plainly and correctly bearing the name, address, and telephone number of the intended recipient in the United States; and shall plainly and correctly bear on the outer container the following information:
(b) Any terrestrial plant which is to be exported or reexported by mail and which may be exported or reexported under 50 CFR part 17 or part 23 only if accompanied by documentation, shall be mailed to Plant Protection and Quarantine (at a port authorized for such purpose by the U.S. Department of the Interior in 50 CFR part 24 pursuant to section 9(f) of the Act (16 U.S.C. 1538(f))); shall be wrapped in double wrapping, with an unsealed inner wrapping addressed to the foreign recipient and bearing sufficient postage for mailing to the foreign destination; shall be accompanied by a separate sheet of paper within the package plainly and correctly bearing the following information:
(4) Serial number and type (e.g. permit, certificate) of document issued for the exportation or reexportation of the plant.
(Information collection requirements were approved by the Office of Budget and Management under control number 0579-0076)