I am an American living in Spain illegally.  I teach mostly private English classes, but have a Spanish autonomo friend who is interested in paying me to help her with her website.  I need to be able to invoice her, but as I am not a Spanish resident, I don’t have an NIE.  I understand that many Americans open businesses in the US so that they can invoice in Spain.  Does anyone have any experience with this that they would be willing to share with me. 

Many Thanks!