In order to determine whether, as a condition for the exemption of the supply of goods carried in the personal luggage of travellers, the threshold set by a Member State in accordance with point (c) of the first subparagraph of Article 147(1) of Directive 2006/112/EC has been exceeded, the calculation shall be based on the invoice value. The aggregate value of several goods may be used only if all those goods are included on the same invoice issued by the same taxable person supplying goods to the same customer.