National Home Gardening Club sent back last book (vegetable gardening ) after receiving it(03/12/12) now they're threatening me(past due) Hopkins, Minnesota

I ordered the first book from them in 2011 along with the free gifts.  One was a hand shovel which broke immediatly.  They did send a new one which was broken in a short time.  The last book received was a book on Vegetable Gardening. ( 3/12/12) I returned it to them within a week unopened.  Now, saying it is past due. They keep sending me a bill on something returned to them.  Now they say in the last bill I owe them $39.98.on that book. I have not received any more books from them.  But, still owe them for the last one.  This harresment has to stop.