Customs duty on polyester film amended to add a specific per kilogram charge alongside existing ad valorem rate. The Central Government, invoking powers under section 25(1) of the Customs Act, 1962, amends Notification No. 87/89 Customs to substitute the previous '40% ad valorem' duty for polyester film with '40% ad valorem plus a specific per kilogram duty', thereby changing the duty basis for polyester film in Chapter 39.