Longulf Trading interview questions

XYZ Trading company purchased 1,000 metric tons of HDPE blow moulding resins from ABC Trading @ US$ 1200 per mt DAP Milton Kynes. Payment terms 90 days from bill of lading date which was 1st of July. Contract states an interest clause at 12% per annum for any delayed payments. XYZ was asked to pay duty. Why? When was the payment due? Customer paid with value date 15th Dec. how much interest was charged?