Via Import Cost Allocations, one can allocate the exact importation cost (taxes, delivery etc.), taxes that aren't generated from the Export Supplier.
The system will then calculate the cost of the items correctly. They can be then allocated automatically across Purchase Order items, or manually allocated to each item.
The process automatically enters the Supplier Invoice for these additional import costs, getting the Producst Costing right. This will avoid later problems with having to amend costs manually.