What Does Dni Mean
What is meant by DNI? Is it for Spanish citizens only?
I think you are right, this is the national point number.
What happens when you enter a port number? Does it matter if he is Dutch?
As the site is in Spanish, it will be the same as your national identity card, which does not mean it must be a Spanish identity document.
Sorry, can't help much.
Can you contact the company you ordered if the port number does not work?