Where do you take a post card to cash it in?

You typically cannot "cash in" a postcard as they are not currency or redeemable for cash. If you're referring to a postcard with a prize or promotion, you would need to follow the specific instructions provided on the postcard, which may involve sending it to a designated address or claiming a prize online. For general inquiries or promotions, it's best to check with the organization that issued the postcard.