Letter.cs

public class Letter : MailPiece 
{
    public override decimal Postage { get; set; }
    public override decimal Width { get; set; }
    public override decimal Height { get; set; }
    public override decimal Thickness { get; set; }

    public Letter()
    {
        Postage = 0.46M;
        Width = 11.5M;
        Height = 6.125M;
        Thickness = 0.25M;
    }
}