What year is this note from? I believe this is in reference to the Comstock Laws, which made it illegal to mail "obscenity," usually the law was used against pornography and sex ed/birth control information, but this is some pretty red hot baseball language, so perhaps they were worried it would fall under that. I'm not sure what company exactly Express is referring too, but presumably the instruction means "don't send this through the USPS, use private courier."