[SerializableAttribute()] 
public class OrderLineFieldValue 

[Subscribe(Dynamicweb.Notifications.eCommerce.Order.Steps.Completed)]
public class OrderCompleteNotificationSubscriber : NotificationSubscriber
{
    public override void OnNotify(string notification, object[] args)
    {
        Dynamicweb.eCommerce.Orders.Order MyOrder = (Dynamicweb.eCommerce.Orders.Order)args[0];
        foreach (Dynamicweb.eCommerce.Orders.OrderLine line in MyOrder.OrderLines)
        {
            foreach (Dynamicweb.eCommerce.Orders.OrderLineFieldValue field in line.OrderLineFieldValues)
            {
                if (field.OrderLineFieldSystemName.ToLower() == "dispatchestimate")
                {
                    field.Value = "Week 45";
                    line.Save();
                    break;
                }
            }
        }
    }
}