Quoted from the SPAMcop list:

If you live in the US, and they try this scam on you by mailing the
directory with a bill, quote or cite 39 USC 3001(d) at them:

(d) Matter otherwise legally acceptable in the mails which--
(1) is in the form of, and reasonably could be interpreted or
construed as, a bill, invoice, or statement of account due; but
(2) constitutes, in fact, a solicitation for the order by the
addressee of goods or services, or both;
is nonmailable matter, shall not be carried or delivered by mail,
and shall be disposed of as the Postal Service directs, unless such
matter bears on its face, in conspicuous and legible type in
contrast by typography, layout, or color with other printing on its
face, in accordance with regulations which the Postal Service shall
prescribe--
(A) the following notice: "This is a solicitation for the order of
goods or services, or both, and not a bill, invoice, or statement
of account due. You are under no obligation to make any payments on
account of this offer unless you accept this offer."; or
(B) in lieu thereof, a notice to the same effect in words which the
Postal Service may prescribe.

When you receive an unsolicited book or package of any kind, write
them a polite note thanking them for the free gift, and quote or
cite 39 USC 3009(b):

(b) Any merchandise mailed in violation of subsection (a) of this
section, or within the exceptions contained therein, may be treated
as a gift by the recipient, who shall have the right to retain, use,
discard, or dispose of it in any manner he sees fit without any
obligation whatsoever to the sender. All such merchandise shall have
attached to it a clear and conspicuous statement informing the
recipient that he may treat the merchandise as a gift to him and has
the right to retain, use, discard, or dispose of it in any manner he
sees fit without any obligation whatsoever to the sender.

However, in my experience, Target Publications got around
the above regulation in two ways.
A. They did not send the invoice WITH the directory, it was sent separately.
B. They sent the directory via a private shipper (FedEx or UPS, I don't recall)
and thus got around the regulation which only applies to material mailed via USPS.