If you are using a Master Page you need to get a reference to the ContentPlaceholder first. Something like

Code:
Confirm.aspx code behind

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        If PreviousPage IsNot Nothing Then
            If PreviousPage.IsCrossPagePostBack Then
                Dim ph As ContentPlaceHolder = CType(PreviousPage.Master.FindControl("cphBodyContent"), ContentPlaceHolder)
                Dim txt As TextBox
                If Not ph Is Nothing Then
                    txt = ph.FindControl("txtQuantity")
                    If Not txt Is Nothing Then
                        lblQuantity.Text = "You have ordered " & txt.Text & " " & PreviousPage.SoftwareName & " items."
                    End If
                End If
            End If
        End If
    End Sub