You do not have to pay for something you didn't order, and in fact, in some states say you can charge the sender for storing the item for them. I'd write to them telling them I didn't order it, and I'm not going to pay for it. If they want it back, they'll have to send me a postage-paid label. And, last but not least, tell them not to send you anything more.
I've had problems with America's Test Kitchens in this way (Cook's Illustrated). If I order a book from them, I'm automatically put on their book of the month club and I receive unsolicited books along with a bill, of course. They are 30-day trial offers, and I have to pay the postage to send them back. I started calling in my order and telling them I didn't want to be added to their mailing list (I do like their books). Even at that, I received another unsolicited book. I called them, mad as a hornet, and I told them I wasn't going to send it back unless they paid the postage AND the gasoline for the 60-mile drive I have to make to the post office and back. They told me to keep the book with their compliments. I haven't ordered any more books from them, but I doubt anything will change.