var isPrint = false;      
  function ExportHTMLAndPrint()        {
            var WebGrid1 = ISGetObject("WebGrid1");
            isPrint = true;
            WebGrid1.ExportGrid("", "HTML", "PORTRAIT");
            
        }
        function OnExportPrintResponse(controlId, status) 
        {
           var grid = ISGetObject(controlId);
            if (isPrint) {
                var fileLocation = ISXml.GetNodeText(grid.XmlResp, "//outputFile");
                var iFrameWindow = document.getElementById("printFrame");
                if (!iFrameWindow) {
                    iFrameWindow = document.createElement("iframe");
                    iFrameWindow.id = "printFrame";
                    iFrameWindow.style.display = "none";
                    document.body.appendChild(iFrameWindow);
                }
                iFrameWindow.src = fileLocation;
                debugger;
                if (iFrameWindow.contentWindow.print) {
                    iFrameWindow.onload = function () { iFrameWindow.contentWindow.print() };
                }
                else {
                    iFrameWindow.onload = iFrameWindow.contentWindow.document.execCommand("print", false, null);
                }
                grid.SetStatus1("", "ready", "CommonText/Ready");
                return false;
        }

 and here is my client side event I intercept the OnResponse

<ClientSideEvents OnResponse="OnExportPrintResponse" />

The result is whenever you press the button it will automatically show print pages using export to HTML.
Hope this will help!