Cost layers

type CostLayer = {
  code: string;         // stable machine token, e.g. "SEC_301", "MPF"
  amount: number;       // dollars in the response currency
  rate: number | null;  // ad valorem rate as a decimal, null for flat fees
  exempted: boolean;    // true when the layer would apply but is waived
  note: string | null;  // optional human-readable annotation
};

total_landed_cost
  = customs_value              (goods + freight + insurance + dutiable adjustments)
  + SUM(duty layer amounts)
  + SUM(tax layer amounts)
  + SUM(fee layer amounts)