We use most-favored nation (MFN) rates for import duty calculations. We do not currently support preferential rates due to many countries requiring a Certificate of Origin (CO) for preferential treatment. Typically, ecommerce shipments do not include a CO so using MFN rates ensures that our results are conservative and accurate.
We are planning to introduce preferential rates at a later date – feel free to contact us if you have any questions regarding when we will launch this feature.