Shopping cart System Project

Hey guys, doing a Shopping Cart System project and i have all the code for it, no errors or anything but everytime i go to try and run it i get an error message stating the following

"Exception thrown: 'System.InvalidCastException' in Microsoft.VisualBasic.dll
An unhandled exception of type 'System.InvalidOperationException' occurred in Shopping Cart.exe
Additional information: An error occurred creating the form. See Exception.InnerException for details. The error is: Conversion from string "Subtotal:" to type 'Double' is not valid."

I dont know what it is but i really could use some help, insight, something!!

Here is the code in case you need it

Thank you in advance!

Main Form

Public Class MainForm
Private Sub MainForm_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
lblTax.Text = "0.06%"
lblShipping.Text = "$2/Book"
End Sub

Public Sub lblSubTotal_TextChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles lblSubtotal.TextChanged
Const SalesTax As Single = 0.06
Const Shipping As Single = 2.0
Dim TotalSale As Single = 0

lblTax.Text = FormatNumber(lblSubtotal.Text * SalesTax) 'calculate & Display SalesTax

lblShipping.Text = FormatCurrency((lbBooks.Items.Count) * Shipping) 'calculate & Display Shipping

TotalSale = FormatNumber(CSng(lblSubtotal.Text) + CSng(lblTax.Text) + CSng(lblShipping.Text))

lblTotal.Text = FormatCurrency(TotalSale)

End Sub

Private Sub btnRemove_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnRemove.Click
'BtnRemove performs one of two tasks:
'1.)Clear selected Item from listBox
'2.)Clear all form Controls
Dim strItem As String = "empty"
strItem = lbBooks.SelectedItem 'Sets strItem to selected item

If strItem = "" Then 'If no Item selected then clear all controls
lbBooks.Items.Clear()
lblSubtotal.Text = 0
lblTax.Text = "0.06%"
lblShipping.Text = "$2/Book"
lblTotal.Text = 0
Else
ReCal(strItem)

lbBooks.Items.Remove(strItem) 'Removes Selected Item from ListBox

'Recalculate the shipping cost after item removal
lblShipping.Text = FormatCurrency((lbBooks.Items.Count) * 2.0)

End If
End Sub

Private Sub ResetToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ResetToolStripMenuItem.Click
'Clear All -- Resets all the form controls to Initial settings
lbBooks.Items.Clear()
lblSubtotal.Text = 0
lblTax.Text = "0.06%"
lblShipping.Text = "$2/Book"
lblTotal.Text = 0

End Sub

Private Sub ExitToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ExitToolStripMenuItem.Click
Me.Close() 'End the program Run
End Sub

Private Sub PrintBooksToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles PrintBooksToolStripMenuItem.Click
PrintBooks.Show() 'Display the Print Books Select Form
End Sub

Private Sub AudioBooksToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles AudioBooksToolStripMenuItem.Click
AudioBooks.Show() 'Display the Audio Books Select Form
End Sub

Private Sub AboutToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles AboutToolStripMenuItem.Click
MessageBox.Show(Me, "Shopping Cart System", "About")
End Sub


End Class

AudioBooks

Public Class AudioBooks
Private Sub AudioBooks_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
'Loads list of book titles into Audio list box
With lbAudioBooks
.Items.Add("I Did It Your Way " & "(Audio)")
.Items.Add("History of Scotland " & "(Audio)")
.Items.Add("Learn Calculus in one Day " & "(Audio)")
.Items.Add("Relaxation Techniques " & "(Audio)")
End With
End Sub
Private Sub btnClose_click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnClose.Click
MainForm.Show()
End Sub

Private Sub btnAdd_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnAdd.Click
Dim strSelect As String = "empty"
Dim strBook As String = "empty"
Dim strMsg As String = "empty"
Dim Price As Single
Dim intX As Integer = 0


Try
strSelect = lbAudioBooks.SelectedItem.ToString
strSelect = strSelect.Substring(0, intX)
GetPrice(strSelect, Price)
MainForm.lbBooks.Items.Add(strSelect & " -- " & Price)

MainForm.lblSubtotal.Text = FormatNumber(Val(MainForm.lblSubtotal.Text) + Price)

Catch
strMsg = "A Book was Not Selected. "
MessageBox.Show(strMsg, "InValid InPut")

End Try
End Sub
End Class

Print Books

Public Class PrintBooks
Private Sub PrintBooks_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
'Loads list of book titles into Print list Box
With lbPrintBooks
.Items.Add("I Did It Your Way " & "(Print)")
.Items.Add("History of Scotland " & "(Print)")
.Items.Add("Learn Calculus in one Day " & "(Print)")
.Items.Add("Feel the Stress " & "(Print)")
End With
End Sub
Private Sub btnClose_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnClose.Click
MainForm.Show() 'Returns to shopping cart form
Me.Hide() 'Makes PrintBK form invisible
End Sub

Public Sub btnAdd_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnAdd.Click
Dim strSelect As String = "empty"
Dim strBook As String = "empty"
Dim strMsg As String = "empty"
Dim Price As Single
Dim intX As Integer = 0

'Try ~ Catch code prevents a fault if the Add button is clicked with no book selected.
Try
strSelect = lbPrintBooks.SelectedItem.ToString 'Length Book Title + (Print
strSelect = strSelect.Substring(0, intX)

GetPrice(strSelect, Price)
MainForm.lbBooks.Items.Add(strSelect & " -- " & Price)

MainForm.lblSubtotal.Text = FormatNumber(Val(MainForm.lblSubtotal.Text) + Price)

Catch
strMsg = "A Book was Not Selected. "
MessageBox.Show(strMsg, "InValid InPut")
End Try
End Sub
End Class

pricing module

Module PriceSelect
Public Sub GetPrice(ByRef Book As String, ByRef BPrice As Single)

Select Case Book
Case "I Did It Your Way"
BPrice = 11.5
Case "History of Scotland"
BPrice = 14.5
Case "Learn Calculus in one Day"
BPrice = 29.95
Case "Feel the Stress"
BPrice = 18.5
Case "Relaxation Techniques"
BPrice = 14.5
Case "The Science of Body Language"
BPrice = 12.95
Case Else
MessageBox.Show("Book Not Found", "InValid Selection")
End Select

End Sub
End Module


recalc module
Module Recalculate
Public Sub ReCal(ByRef strX As String)
Dim sngSubTotal As Single
Dim SPrice As Single
Dim intR
sngSubTotal = MainForm.lblSubtotal.Text
intR = CInt(strX.Length - 5)
SPrice = strX.Substring(intR, 5)

sngSubTotal = sngSubTotal - SPrice


MainForm.lblSubtotal.Text = FormatNumber(Val(sngSubTotal))

End Sub
End Module



Thats all the code i have please help! Thanks