Dealing with non-inherited methods in a subclass
public abstract class Voucher
    {
        public int Id { get; set; }
        public decimal Value { get; protected set; }
        public const string SuccessMessage = "Applied";
    }

public class GiftVoucher : Voucher
    {
    }

public class DiscountVoucher : Voucher
    {
        public decimal Threshold { get; private set; }
        public string FailureMessage { get { return "Please spend £{0} to use this discount"; } }
    }

if (voucher is DiscountVoucher)
{
   // cast voucher to a DiscountVoucher and then call the specific methods on it
}

public virtual string FailureMessage { get { return string.Empty; } }

public abstract class Voucher
{
    public int Id { get; set; }
    public decimal Value { get; protected set; }
    public const string SuccessMessage = "Applied";
    public decimal Threshold { get { return 0.0; } }
    public string FailureMessage { get { return ""; } }
}

public abstract class Voucher
{
    public bool isValid(ShoppingCart sc);
    public string FailureMessage { get { return "This voucher does not apply"; } }
    // ...
}

public class DiscountVoucher : Voucher
{
    private decimal Threshold;
    public override bool isValid(ShoppingCart sc)
    {
        return (sc.total >= Threshold);
    }
    public override string FailureMessage
    {
        get { return FormatString("Please spend £{0} to use this discount", Threshold); }
    }

public abstract class Voucher
{
    public int Id { get; set; }
    public decimal Value { get; protected set; }
    public virtual string SuccessMessage { get { return "Applied"; } }
    public virtual string FailureMessage { get { return String.Empty; } }
    public virtual bool Ok { get { return true; } }
}

public class GiftVoucher : Voucher { }

public class DiscountVoucher : Voucher
{
    public decimal Threshold { get; private set; }
    public override string FailureMessage { get { return "Please spend £{0} to use this discount"; } }
    public override bool Ok { get { return Value >= Threshold; } }
}

if (voucher.Ok) {
    Console.WriteLine(voucher.SuccessMessage);
} else {
    Console.WriteLine(voucher.FailureMessage);
}