IGST on high sea sales is collected only at importation, with value additions across transactions included in assessable value. IGST on high sea sales of imported goods is collectable only at the time of importation when import declarations are filed; value additions from each high sea sale form part of the value on which IGST is assessed. The importer clearing the goods must furnish the complete chain of documents-original invoice, high sea sale contract, and service/commission details-to support the declared value, and Customs may reject the declared value and determine valuation under the Customs Valuation Rules if doubts arise.