/// <summary>
/// adds items to the order
/// </summary>
/// <param name="item"> the item being added</param>
public void Add(IOrderItem item)
{
currentListIndex = orders.Count;
parent.SendButt.IsEnabled = false;
orders.Add(item);
_subTotal += item.Price;
_totalCalories += item.Calories;
NotifyPropertyChanged("_subTotal");
NotifyPropertyChanged("_tax");
NotifyPropertyChanged("_total");
NotifyPropertyChanged("_totalCalories");
parent.swapScreens();
}