Integrated GST on high sea sales collected only at importation; value additions included and chain documents required by importer. IGST on high sea sale transactions of imported goods is to be levied and collected only at the time of importation when import declarations are filed; value additions from each high sea sale must be included in the value for IGST. The importer/last buyer must furnish the chain of documents (original invoice, high-seas-sales contract, service charges/commission details) to establish value linkage, and the department may reject declared transaction value and determine price under Customs Valuation rules where doubt exists.