That's right. In the U.S., if somebody sends you Unsolicited Goods, you not only have no obligation to pay for it, but you also have the right to keep it.
If you drop a card in the box for one of those "Free Trial Offers", then the understanding is, you'll accept the product, without obligation, to see if you want more.
Don't want more? Like the folks have said, you just write "CANCEL" across the invoice and send it back. End of story!