public interface IWithPrice 

public class MyDiscountProvider : Dynamicweb.eCommerce.Orders.SalesDiscounts.SalesDiscountProvider
{
    private OrderLine CreateOrderLine(Order order)
    {
        Dynamicweb.eCommerce.Prices.PriceInfo priceInfo = ((Dynamicweb.eCommerce.Prices.IWithPrice)order.OrderLines).Price;

        ////Add a new order line
        OrderLine line = new OrderLine();
        line.Order = order;
        line.Quantity = 1;
        line.ProductName = this.DiscountName;
        line.SetUnitPrice(priceInfo);
        line.ParentLineID = null;
        line.DiscountID = DiscountID;

        if (IsProductDiscount)
            line.Type = Base.ChkString(Base.ChkNumber(OrderLine.OrderLineType.ProductDiscount));
        else
            line.Type = Base.ChkString(Base.ChkNumber(OrderLine.OrderLineType.Discount));

        return line;
    }
}