The NIE application form is the EX-15, and the fee for assignment of a NIE is 9.84 euros. The EX-18 is a different form: it is the EU citizen registration certificate application, under a separate procedure, and turning up with it instead is a common reason for being sent away. The NIE is usually the first step, because the utility and council registrations depend on it.
Where this comes from: Asignacion de NIE a instancia de interesado, Policia Nacional sede electronica, read 2026-09-07; Orden PJC/617/2025 de 13 de junio, annex 5.12, in force 16 June 2025, BOE consolidated text, read 2026-09-07.