Customs liability extended to exporters by amendment, placing exporters within the proviso's regulatory scope alongside importers. Insertion of 'or the exporter, as the case may be' after 'importer' in clause (a) of the first proviso to section 27(2) extends the proviso's applicability to exporters, subjecting exporters to the same procedural and substantive effects, obligations, or safeguards that the proviso applies to importers.