How to produce message box in Update Panel in ASP.NET ?

Hi experts!

I’m professional programmer from 10+ years, but new to ASP.NET. I have a easy question to you: how to produce message box to user in ASP.NET ? But satiation is little more complex.

I have a gridview and after user click on button on this gridview it goes to code behind and here I need to produce message to customer.

And to question not be so  easy, this gridview is in UpdatePanel. Till now I was use in Submit buttons this:
Page.RegisterStartupScript("codice", "<script>alert('Accept!'" + customerID + "' );</" + "script>");

But look that this not work in UpdatePanel.

Question 2:
 And other question – do I have options to call Input box in which user can enter some info ? What are command in C# ?

Question 3:
 What are command to open new popup page in code after user click button on some row in GridView.


        protected void grdInvoice_RowCommand(object sender, GridViewCommandEventArgs e)
        {
            string customerID = (string)e.CommandArgument;


            if (e.CommandName == "Accept")
            {
               
                Page.RegisterStartupScript("codice", "<script>alert('Accept!'" + customerID + "' );</" + "script>");
                //AcceptFunction(customerID);
            }
            else if (e.CommandName == "Deny")
            {
                Page.RegisterStartupScript("codice", "<script>alert('Deny!'" + customerID + "' );</" + "script>");
            }


        }