gui help

I can not figure out how to add a way to get the following program to
display the total value of the inventory or a re-stocking fee. Any help
would be greatly appreciated.

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;
class Testing
{
  java.util.List dvds = new java.util.ArrayList();

  JTextField tfTitle = new JTextField(15);
  JTextField tfProductNumber = new JTextField();
  JTextField tfPrice = new JTextField();
  JTextField tfQuantity = new JTextField();
  DefaultListModel dlm = new DefaultListModel();
  JList list = new JList(dlm);
  public void buildGUI()
  {
    JButton btn = new JButton("Add");
    JPanel p1 = new JPanel(new BorderLayout());
    JPanel p = new JPanel(new GridLayout(4,2));
    p.add(new JLabel("DVD Title: "));
    p.add(tfTitle);
    p.add(new JLabel("Product Number: "));
    p.add(tfProductNumber);
    p.add(new JLabel("Price per Unit: "));
    p.add(tfPrice);
    p.add(new JLabel("Quantity on Hand: "));
    p.add(tfQuantity);
    p1.add(p,BorderLayout.NORTH);
    JPanel p2 = new JPanel();
    p2.add(btn);
    p1.add(p2,BorderLayout.CENTER);
    JFrame f = new JFrame();
    f.getContentPane().add(p1,BorderLayout.NORTH);
    JScrollPane sp = new JScrollPane(list);
    f.getContentPane().add(sp,BorderLayout.CENTER);
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    f.pack();
    f.setLocationRelativeTo(null);
    f.setVisible(true);
    btn.addActionListener(new ActionListener(){
      public void actionPerformed(ActionEvent ae){
        dvds.add(new DVD(tfTitle.getText(),tfProductNumber.getText(),
                                Double.parseDouble(tfPrice.getText()),

Integer.parseInt(tfQuantity.getText())));
        setList();
        clearTextFields();
      }
    });
    list.addListSelectionListener(new ListSelectionListener(){
      public void valueChanged(ListSelectionEvent lse){
        if(lse.getValueIsAdjusting() == false)
        {
          DVD dvd = (DVD)dvds.get(list.getSelectedIndex());
          tfTitle.setText(dvd.title);
          tfProductNumber.setText(dvd.productNumber);
          tfPrice.setText(""+dvd.price);
          tfQuantity.setText(""+dvd.quantity);
        }
      }
    });
  }
  public void setList()
  {
    dlm.clear();
    for(int x = 0, y = dvds.size(); x < y; x++)
    {
      dlm.addElement((DVD)dvds.get(x));
    }
  }
  public void clearTextFields()
  {
    tfTitle.setText("");
    tfProductNumber.setText("");
    tfPrice.setText("");
    tfQuantity.setText("");
    tfTitle.requestFocusInWindow();
  }
  public static void main(String[] args)
  {
    EventQueue.invokeLater(new Runnable(){
      public void run(){
        new Testing().buildGUI();
      }
    });
  }
}
class DVD
{
  String title;
  String productNumber;
  double price;
  int quantity;
  public DVD(String t,String pn, double p, int q)
  {
    title = t; productNumber = pn; price = p; quantity = q;
  }
  public String toString(){return title;}
}