Federal Law
Federal law prohibits any mailing which is “in the form of, and reasonably could be interpreted or construed as, a bill, invoice, or statement of account due” but is, in fact, “a solicitation for the order by the addressee of goods or services,” unless the mailing includes 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.
Federal law requires that this notice (or one like it) be printed on the face of the mailing in “conspicuous and legible type in contrast by typography, layout, or color with other printing on its face.”
The Minnesota Attorney General's Office sums it up nicely.
Ref:
Minnesota Attorney General's Office on Fake InvoicesIt never hurts to read up on your rights ;)