How do I properly use checkboxes for running total in VB.net?

I'm a complete newbie when it comes to programming in VB.net (started last week). I have an assignment in class that requires me to do a running total using 3 checkboxes.

I've figured out a little bit of it. My main problem right now is that when I uncheck boxes, the calculation gets weird. So far here is my code:

Public Class frm1
    Dim HandbagAcc As Double = 0

    Private Sub chkBoxExtra_CheckedChanged(sender As Object, e As EventArgs) Handles chkBoxExtra.CheckedChanged
        '$9.99
        If chkBoxExtra.Checked = True Then
            HandbagAcc = HandbagAcc + 9.99
            txtBoxAcc.Text = FormatCurrency(HandbagAcc)
        End If
        If chkBoxExtra.Checked = False Then
            HandbagAcc = HandbagAcc - 9.99
            txtBoxAcc.Text = FormatCurrency(HandbagAcc)
        End If
    End Sub

    Private Sub chkBoxMatching_CheckedChanged(sender As Object, e As EventArgs) Handles chkBoxMatching.CheckedChanged
        '$7.99
        If chkBoxExtra.Checked = True Then
            HandbagAcc = HandbagAcc + 7.99
            txtBoxAcc.Text = FormatCurrency(HandbagAcc)
        End If

        If chkBoxExtra.Checked = False Then
            HandbagAcc = HandbagAcc - 7.99
            txtBoxAcc.Text = FormatCurrency(HandbagAcc)
        End If
    End Sub
    Private Sub chkBoxFour_CheckedChanged(sender As Object, e As EventArgs) Handles chkBoxFour.CheckedChanged
        '$3.95
        If chkBoxExtra.Checked = True Then
            HandbagAcc = HandbagAcc + 3.95
            txtBoxAcc.Text = FormatCurrency(HandbagAcc)
        End If

        If chkBoxExtra.Checked = False Then
            HandbagAcc = (HandbagAcc - 3.95)
            txtBoxAcc.Text = FormatCurrency(HandbagAcc)
        End If
    End Sub
End Class

When I check the three checkboxes the first time, the values add up nicely--but, when I uncheck a box or two, my numbers go wonky and the calculation doesn't add up. What am I doing wrong?

Any help would be much appreciated. Thank you!