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(c) Any restricted article for importation (by mail or otherwise), at the time of importation or offer for importation into the United States shall be accompanied by an invoice or packing list indicating the scientific names of the articles, at least to the level of genus, and the quantity of plants for planting in the shipment. Quantity must be expressed in the number of plant units, or in the case of seeds, by weight in grams or kilograms. Finally, when the regulations in this subpart place restrictions on individual species or cultivars within a genus, the invoice or packing list must also identify the species or cultivar of the articles.