The guy was outside the door? He told you the last name and even signed it and it was the correct one on the package? As long as you didn't sign it yourself and you asked for his clarification, you followed the methods.
Are we supposed to ID everyone every time we get a signature? We'd never get done. You did your job. Be calm, explain you followed the methods and that the man was standing outside the residence and gave you the name on the package and signed it without looking at the package first.
Not too hard to prove you did nothing wrong. You should be fine