Which type of invoice is commonly used by customs to determine the value of goods for duties?

The commercial invoice is essential in international trade as it serves as the primary document that customs officials use to assess the value of goods being shipped for the purpose of calculating duties and taxes. This invoice includes detailed descriptions of the items, their value, and relevant terms of sale, which provide customs authorities with the necessary information to determine whether the declared value aligns with their regulations.

The proforma invoice, while it may indicate potential costs and is often used as a preliminary bill before the actual sale, does not reflect the final transaction details and therefore is not relied upon for duty valuation. A standard invoice is typically used in domestic transactions and lacks the specific requirements necessary for customs processes. A credit invoice pertains to adjustments or returns in accounting and likewise does not fulfill the role needed for customs duty evaluations.

Therefore, the commercial invoice stands out as the correct answer due to its critical role in the customs clearance process and its direct relevance to the valuation of goods for duty assessment.