You can alert the user using windows load function.
window.onload = function () {
   alert('Package Successfully Booked!');
}
or you can do it this way
<html>
<head>
<script>
function showAlert()
    {
    alert("Package Successfully Booked!");
    }
    </script>
</head>
<body onload="showAlert()">
</body>
</html>
solved Javascript alert and back with php