Solution: E-Commerce Shipping Calculator

namespace Retail;
public class PostalShipment
{
    public double WeightInPounds { get; init; }
    public virtual decimal CalculateCost()
    {
        return 5.00m + (decimal)(WeightInPounds * 0.5);
    }
}
public class ExpressShipment : PostalShipment
{
    public override decimal CalculateCost()
    {
        return base.CalculateCost() + 10.00m;
    }
}