It is a case of composite supply. GST on freight will also be paid at the rate of GST applicable to principal supply which is 18% in your case. Principal supply attracts 18% as mentioned by you. They cannot be separated. The tax rate of the principal supply will apply on the entire supply.
Illustration :
See the definition of composite supply
Section 2 (30) of CGST Act “composite supply” means a supply made by a taxable person to a recipient consisting of two or more taxable supplies of goods or services or both, or any combination thereof, which are naturally bundled and supplied in conjunction with each other in the ordinary course of business, one of which is a principal supply;
Illustration. - Where goods are packed and transported with insurance, the supply of goods, packing materials, transport and insurance is a composite supply and supply of goods is a principal supply;
Both the supplies cannot be separated. The tax rate of the principal supply will apply on the entire supply.