The right thing to do is to ask them. I surprised no one else has chimed in on this. It was a mistake that you received the extra motors. You shouldn't be profiting from it.

I can speak from first hand experience that CC has excellent customer service that typically extends past what they would legally be responsible to provide. For example, replacing a part part even though the warranty has expired. They deserve the opportunity to receive back the merchandise they sent you by mistake.

On the other hand, if you do reach out to them and they tell you to not worry about it, then no harm done.

ben