US postal regulations are quite clear that you are NOT OBLIGATED to pay for or even to return "unordered merchandise."
I've received lots fewer of them since that reg went into effect.
If you didn't order it, it's yours, for free, and I've even stopped sending them a thank you. Including a fictitious "bill" has no effect (on me), although I might send the bill back with notation that the "merchandise" was not ordered, since my notice might be required if I need to contest a claim on a credit record.
For an otherwise respectable source I might offer to return the item as soon as they sent me a "Return Merchandise Authorization" and a suitable shipping container with pre-paid postage. My experience has been that they almost never request the return.
The problem with this scam is that they've found a way to charge your credit card before they inform you that they've "delivered" something, so you have the problem of getting the credit card charges removed. And it appears that in some cases you may not even be informed that you've "received" a "membership" unless you check your credit card statement.