Rejection of declared value can trigger sequential valuation methods if customs reasonably doubts declared export value. When a proper officer reasonably doubts the truth or accuracy of an exporter's declared value, the officer may request further information and, if doubts persist or there is no response, treat the transaction value as not determined and proceed to alternative valuation methods. The officer must, on request, provide written grounds for doubt and a reasonable opportunity of being heard before finally rejecting the declared value. The rule establishes procedure, not a valuation method, and cites grounds for doubt such as significant value variation, values above market, and misdeclaration of goods.