By complying.........you admit guilt. You responded once (which you shouldn't have) and they are going to make you more afraid. Unless the letter admonishes you from future lawsuits, it is just plain extortion.
If you went to the actual company, gave them a check and dropped of any merchandise, they wouldn't know what you were talking about.
Unless you get a certified letter from a lawyer actually working for 20th century fox, don't do anything.