Having trouble with error checking in Select Case

If lstWorkshop.SelectedItem Is Nothing And lstLocation.SelectedItem Is Nothing Then
    MessageBox.Show("Select both a workshop and a location")
    Return
End If

If lstWorkshop.SelectedItem Is Nothing Or lstLocation.SelectedItem Is Nothing Then
    MessageBox.Show("Select both a workshop and a location")
    Return
End If

'Option Strict On

Public Class FormWorkshopSelector

   

    Private Sub btnAddWorkshop_Click(sender As Object, e As EventArgs) Handles btnAddWorkshop.Click

        'Declare variables and assign values for both listboxes
        Dim intFee As Integer
        Dim intTotal As Integer
        Dim intDay As Integer
        Dim intStay As Integer
        Dim Workshop As String = lstWorkshop.SelectedItem
        Dim Location As String = lstLocation.SelectedItem

        If lstWorkshop.SelectedItem Is Nothing Or lstLocation.SelectedItem Is Nothing Then
            MessageBox.Show("Select both a workshop and a location")
            Return
        End If

            ' Select workshop
            Select Case Workshop
            Case CStr("Handling Stress")
                intDay = 3
                intFee = 595
            Case CStr("Time Management")
                intDay = 3
                intFee = 695
            Case CStr("Supervision Skills")
                intDay = 3
                intFee = 995
            Case CStr("Negotiation")
                intDay = 5
                intFee = 1295
            Case CStr("How to Interview")
                intDay = 1
                intFee = 395

        End Select

            ' Select location.
            Select Case Location
                Case CStr("Austin")
                    intStay = intDay * 95
                Case CStr("Chicago")
                    intStay = intDay * 125
                Case CStr("Dallas")
                    intStay = intDay * 110
                Case CStr("Orlando")
                    intStay = intDay * 100
                Case CStr("Phoenix")
                    intStay = intDay * 92
                Case CStr("Raleigh")
                    intStay = intDay * 90
           
        End Select
      

            'Cost of each selected workshop and location
            intTotal = intStay + intFee

            'Display cost of workshop and location in "List Cost" list box
        lstCost.Items.Add(intTotal.ToString("c"))
       

    End Sub

    Private Sub btnCalcTotal_Click(sender As Object, e As EventArgs) Handles btnCalcTotal.Click
       
        Dim dblTotal As Double = 0.0   'Accumulator
        Dim sum As Double           ' 

        For x As Integer = 0 To lstCost.Items.Count - 1
            sum += CDbl(lstCost.Items(x))
        Next
        lblTotalCost.Text = sum.ToString("c")
       

    End Sub

    Private Sub btnReset_Click(sender As Object, e As EventArgs) Handles btnReset.Click
        'Reset the Workshop and Location list boxes.
        lstWorkshop.SelectedIndex = -1
        lstLocation.SelectedIndex = -1

        'Reset the List of Costs list box.
        lstCost.Items.Clear()

        'Reset the Total Cost label
        lblTotalCost.Text = String.Empty

        ' Return the focus to Pick a Workshop.
        lstWorkshop.Focus()
    End Sub
    Private Sub FormWorkshopSelector_FormClosing(sender As Object, e As FormClosingEventArgs) Handles MyBase.FormClosing
        ' Verify closing of form.
        If MessageBox.Show("Are you sure you want to exit?", "Confirm",
                           MessageBoxButtons.YesNo) = DialogResult.Yes Then
            ' Continue to close the form.
            e.Cancel = False
        Else
            ' Do not close the form.
            e.Cancel = True
        End If
    End Sub
    Private Sub btnExit_Click(sender As Object, e As EventArgs) Handles btnExit.Click
        ' Close the form.
        Me.Close()
    End Sub

    
End Class

intTotal = intStay(warn) + intFee(warn)

Dim Workshop As String = lstWorkshop.SelectedItem.ToString
Dim Location As String = lstLocation.SelectedItem.ToString

Select Case lstWorkshop.SelectedIndex
    Case -1
        MsgBox("Select 2 values, man!")
        Exit Sub
    Case 0
...
...
...
...
End Select

Select Case lstLocation.SelectedIndex
    Case -1
        MsgBox("Select 2 values, man!")
        Exit Sub
    Case 0
...
...
...
...
End Select