The products of the territory of any contracting party imported into the territory of any other contracting party shall be accorded treatment no less favourable than that accorded to like products of national origin in respect of all laws,regulations and requirements affecting their internal sale,offering for sale,purchase,transportation,distribution or use. The provisions of this paragraph shall not prevent the application of differential internal transportantion charges which are based exclusively on the economic operation of the means of transport and not on the nationality of the product.