It's more wordplay- pornographic materials couldn't be sent through the mails for a long time, so they're implying that the girl is so erotic she's almost pornographic.
darquegk and PJ are correct. Most places in the U.S. had laws against disseminating obscene material until Miller v. California in 1973. (Think of the way child porn is treated even now.)
Hence, "the kind of girl you can't send through the mails".
I believe there were special federal laws that covered obscene material sent via mail or over state lines, which is why it's "can't send through the mails" rather than "will land you in jails."
One more reason why COMPANY should be left in 1970...
I saw the original Broadway production and many other others, and have listened to the album hundreds of times since I was a teenager. I certainly remember the Supreme Court obscenity rulings of the 1970s.
But, coincidentally, I was listening to the OBC recording again last week and for the first time realized what that line meant.