Searching second listbox

'Project Name:              Ship With Us
'Project Purpose:           Input Zip Code and find the associated shipping cost
'Created/Revised by:        

Option Explicit On
Option Strict Off
Option Infer Off

Public Class MainForm

    Private Sub MainForm_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
        'fills the list box with zip codes
        shippingoneListBox.Items.Add("60611")
        shippingoneListBox.Items.Add("60234")
        shippingoneListBox.Items.Add("56789")
        shippingoneListBox.Items.Add("23467")
        shippingoneListBox.Items.Add("60543")
        shippingoneListBox.Items.Add("60561")
        shippingoneListBox.Items.Add("55905")
        shippingoneListBox.Items.Add("89567")
        'fills the list box with second set of zip codes
        shippingtwoListBox.Items.Add("50978")
        shippingtwoListBox.Items.Add("78432")
        shippingtwoListBox.Items.Add("98432")
        shippingtwoListBox.Items.Add("97654")
        shippingtwoListBox.Items.Add("20245")
    End Sub

    Private Sub exitButton_Click(sender As Object, e As System.EventArgs) Handles exitButton.Click
        Me.Close()
    End Sub

    Private Sub displayshipButton_Click(sender As Object, e As System.EventArgs) Handles displayshipButton.Click
        'determine whether a list box contains a specific zip code
        Dim id As String = String.Empty
        'if the user picks a zip code in ListBox1 then the shipping charge is $15.00
        'and if zip code is chose in  ListBox2  shipping charge is $20.00

        If (shippingoneListBox.FindString(zipTextBox.Text >= 0)) Then
            shippingLabel.Text = "Shipping is $15"
            shippingLabel.Show()
        ElseIf (shippingtwoListBox.FindString(zipTextBox.Text >= 0)) Then
            shippingLabel.Text = "Shipping is $20"
            shippingLabel.Show()
        Else
            shippingLabel.Text = "The zipcode was not found!"
            shippingLabel.Show()
        End If
    End Sub
End Class

When entering a zip code that is listed in "shippingtwoListBox" it will still tell me the shipping cost should be $15 when in fact it should be $20. Do I have something coded wrong that it isn't searching the second list box? Also, when I enter a zip code that isn't added to the listboxes the label isn't displaying "The zipcode was not found!"

Any help would be much appreciated!

Tags: Visual Basic, Visual Studio