What is the difference between ad valorem and specific duty under postal rules?

Under the 2025 postal duty rules, carriers choose one method per remittance period. Ad valorem: duty equals the IEEPA tariff rate for the product's origin country, based on product value. Specific: a flat duty of $80-$200 per item depending on the applicable tariff rate. From February 2026, only ad valorem may be used.