My program wont add my constants to the total. It only adds the textbox values i enter for labour and parts?

    public Form1()
    {
        InitializeComponent();
    }

    private void checkBoxOilChange_CheckedChanged(object sender, EventArgs e) // Calls total value passing oil_change as an argument.
    {

        total += OIL_CHANGE;                     // if oil change checkbox status changes add oil change value to total
    }


    private void checkBoxLubeJob_CheckedChanged(object sender, EventArgs e) // Calls total value passing LUBE_JOB as an argument.
    {

        total += LUBE_JOB;                                                   /* if lube job checkbox status changes add lube job
                                                                                  value to total */
    }

    private void checkBoxRadiatorFlush_CheckedChanged(object sender, EventArgs e) // Calls total value passing radiator flush as an argument.
    {

        total += RADIATOR_FLUSH;                                                  /* if radiator flush checkbox status changes add radiator flush
                                                                                  value to total */
    }

    private void checkBoxTransmissionFlush_CheckedChanged(object sender, EventArgs e) // Calls total value passing transmission_flush as an argument.
    {
        total += TRANSMISSION_FLUSH;                                                    /* if transmission flush checkbox status changes add transmission flush
                                                                                            value to total */
    }

    private void checkBoxInspection_CheckedChanged(object sender, EventArgs e) // Calls total value passing Inspection as an argument.
    {
        total += INSPECTION;                                                     /* if inspection checkbox status changes add inspection
                                                                                            value to total */
    }

    private void checkBoxMuffler_CheckedChanged(object sender, EventArgs e)   // Calls total value passing replace muffler as an argument.
    {
        total += REPLACE_MUFFLER;                                              /* if muffler checkbox status changes add muffler
                                                                                            value to total */
    }

    private void checkBoxTyreRot_CheckedChanged(object sender, EventArgs e)  // Calls total value passing tyre rotation as an argument.
    {
        total += TYRE_ROTATION;                                               /* if tyre rotation checkbox status changes add tyre rotaion
                                                                                            value to total */
    }

    private void buttonCalculate_Click(object sender, EventArgs e)
    {
            serviceslabour = decimal.Parse(textBoxLabour.Text) + total;
            parts = decimal.Parse(textBoxParts.Text);
            tax = TAX * parts;
            fees = total + parts + tax + serviceslabour;
            labelOutputServiceLabour.Text = serviceslabour.ToString("n2");
            labelOutputParts.Text = parts.ToString("n2");
            labelOutputTax.Text = tax.ToString("n2");
            labelOutputTotalFees.Text = fees.ToString("n2");
    }