text from code-behind won't display in ModalPopupExtender label

I've been through all the articles the last few hours trying to make dynamic label text display in a label contained in a ModalPopupExtender, with no success. A label text in the markup code works but I can't bring in anything from code-behind. Help, please; this is getting very frustrating. Suggestions have included everything from update panels to JavaScript; maybe I'll just conclude that my ModalPopupExtender is cursed. Please help me lift the curse. Code:

<style type="text/css">
        .modalBackground
        {
            background-color: Gray;
            filter: alpha(opacity=70);
            opacity: 0.7;
        }
        .modalPopup
        {
            background-color: #ffffdd;
            border-width: 3px;
            border-style: solid;
            border-color: Gray;
            padding: 3px;
            width: 250px;
        }</style><script type="text/javascript">
    function ShowModalPopup() { var modal = $find("<%= mpeNewInviteSuccess.ClientID %>"); modal.show(); }</script><%--success popup items--%><div style="display: none;"><asp:Button ID="btnDummy" OnClick="btnDummy_Click" OnClientClick="ShowModalPopup" UseSubmitBehavior="true" runat="server" /></div><div id="divNewInviteSuccess" class="modalPopup" runat="server"><div><asp:Label ID="lblNewInviteSuccess" runat="server" /></div><br /><div><asp:Button ID="btnNewInviteReturn" Text="OK" CssClass="button" OnClientClick="return HideModalPopup()" runat="server" /></div></div><act:ModalPopupExtender ID="mpeNewInviteSuccess" TargetControlID="btnDummy" BackgroundCssClass="modalBackground"
        PopupControlID="divNewInviteSuccess" DropShadow="true" BehaviorID="mpe" runat="server" />



    protected void btnDummy_Click(object sender, EventArgs e)
    {
        mpeNewInviteSuccess.Show();
        //create object of button from sender
        Button button = sender as Button;

        //get command argument
        string cmdArgument = button.CommandArgument;

        //split command argument based on comma separator and convert to a list
        List<string> lstPassedValues = new List<string>(cmdArgument.Split(','));

        //ListView lsv = Helpers.FindChild(Page, "lsvNewInvite") as ListView;

        //create successful message from list
        string SuccessMsg = "Success! Your invitation was sent to " + lstPassedValues[0] + " " + lstPassedValues[1] + " (" + lstPassedValues[2] + ").";
        lblNewInviteSuccess.Text = SuccessMsg;
        mpeNewInviteSuccess.Show();
    }


    protected void lsvNewInvite_ItemCommand(object sender, ListViewCommandEventArgs e)
    {
        if (e.CommandName == "Insert")
        {
            //place cursor in txtFirstName
            var lsv = Helpers.FindChild(this, "lsvNewInvite") as ListView;
            TextBox txt = Helpers.FindChild(lsv, "txtFriendFirstName") as TextBox;
            if (txt != null)
                txt.Focus();

            //set up parameter values
            ListViewItem item = e.Item as ListViewItem;
            var userInfoList = UserInfo.UserInfoGetByUserID(UserID);
            UserInformation = userInfoList.FirstOrDefault();
            var user = UserInformation.UserID;
            string firstName = (item.FindControl("txtFriendFirstName") as TextBox).Text.ToString();
            string lastName = (item.FindControl("txtFriendLastName") as TextBox).Text.ToString();
            string eMail = (item.FindControl("txtFriendEmail") as TextBox).Text.ToString();

            try
            {
                //code to be inserted into DB
                srcNewInvite.InsertParameters["FriendID"].DefaultValue = "0";
                srcNewInvite.InsertParameters["User"].DefaultValue = user.ToString();
                srcNewInvite.InsertParameters["FirstName"].DefaultValue = firstName;
                srcNewInvite.InsertParameters["LastName"].DefaultValue = lastName;
                srcNewInvite.InsertParameters["Email"].DefaultValue = eMail;
                srcNewInvite.InsertParameters["Date"].DefaultValue = DateTime.Now.ToString();
            }
            catch (Exception ex)
            {
                throw ex;
            }
            finally
            {
                EmailNewInvitee(firstName, lastName, eMail);

                //concatenate these values and make it as a comma-separated string
                string concatenatedValue = firstName + "," + lastName + "," + eMail;
                btnDummy.CommandArgument = concatenatedValue;
                //show success message by programatically "clicking" hidden button
                //trigger the button click event
                btnDummy_Click(this.btnDummy, e);
                pnlMain.Visible = false;
            }
        }

    }