private void CheckIfExistsCreditCard(string message)
 {
     if (!CreditCards.Any())
     {
         throw new InvalidOperationException(message);
     }
 }
 /// <summary>
 /// Determines whether [has payment details].
 /// </summary>
 /// <returns></returns>
 public bool HasPaymentDetails()
 {
     return(CreditCards != null && CreditCards.Any());
 }