Please help :'( (visual basic)

[Swearwolf]

basically this is due in today, but im stuck.
its a pizza ordering system etc
i want so when i click a button adding a pizza to the main box, it updates the price aswell.
but when i do, it adds the price for the first item, but doesnt change for others. this is what the code for button is atm:

Public Class Pizzachoose
    Private Sub Button1_Click_1(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim subtotal As String
        'Declares subtotal as a variable
        Form1.RichTextBox1.Text &= Environment.NewLine & ComboBox1.Text
        'Makes the text/name of the Pizza or item go onto the next line
        If ComboBox1.Text = "Cheese and Tomato" Then
            'Finds which pizza has been selected
            subtotal = Val("Form1.TextBox2.Text") + "4.3"
            'Attempts to add the price onto the existing subotal
            Form1.TextBox2.Text = subtotal
            'Displays the new subtotal in the price box
        End If
        'Ends the if funtion
        Me.Close()
        'Closes this sub form
    End Sub
End Class

please help +rep