This post in The Consumerist describes AT&T using a "Receipt Enclosed" solo mail package. While this type of approach can be successful in getting short-term response rates, the fake nature of the package can also have a backlash impact on those who don't respond. Many consumers such as the contributor to Consumerist.com will look at this type of mail as j--k and be less likely to choose AT&T in the future. That's a Fail for Creative.
Lesson: If you want to jeopardize long-term brand equity for short-term results, send a fake important-seeming package.