Unfortunately, we can’t force the customer to award you the prize. It’s their contest and they can change their mind at any time in the 14 days after the Final round ends.
During that time, they might ask friends for their opinions, show the designs to other people in their business, and so on.
Don’t take any promises made by customers too seriously. And don’t take the fact that they wound up picking a different winner too personally. We have plenty of other contests for you to enter.