I would probably call them and tell them of the mistake. Because you are honest and bring this to their attention, if you ever need anything else from them it will be a plus for you. Also, if they made a mistake and shorted you, you would surely tell them, so it has to work both ways. If you both agree you would like to keep the ball, it will probably be a win win situation, as they won't have to issue a call tag and ship another ball. The shipping mistake will probably cost them $40 plus the extra time and aggravation to restock (although it is their mistake) so they may agree to let you keep it.
I worked at a oval track parts business for 11 years and we let some customers keep body parts worth $150 because it would have cost us too much money to get them back. (Truck shipping) Also we sent them the correct parts in addition at no charge and no charge for shipping.
Usually a good business will do what it takes to make the customer happy when the business made a mistake, and in this case, it probably means they will be willing to let you keep a more expensive ball if you agree to it.
One thing to remember in life: My father in law, who was a Michigan State Policeman for 28 years told me this: "In anything you do, you need to be able to look yourself in the mirror in the morning." If you make all your decisions with this in mind, you will be able to sleep well at night.
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