I received a calendar in the mail that I did not order. Am I right that this is a gift that I don't have to pay?

I received a calendar in the mail from a cooking club. I did not order this calendar. They sent a billing statement that if I wanted the calendar to send them the money and if I didn't want to keep the calendar to return it to them. It is my understanding that if a company sends you something that was not ordered that it is considered a gift and that I am not obligated to pay for it or return it.