<script>
    function donate(){
    var id = document.getElementById("steamid").value;
   var rqo = new XMLHttpRequest();
  rqo.open('GET', 'check.php?stid='+id, false);
  rqo.send();
  if (rqo.status != 200) {
  alert('Ошибка' + rqo.status + ': ' + rqo.statusText);
  } else {
   var nomer  = rqo.responseText;
  if(nomer == 'Err'){
  alert('Вы ввели неверные данные!')
  } else {
        var e = document.getElementById("group"),
        selected = e.value,
        res
        var req = new XMLHttpRequest()
        req.open('GET', "pay.php?priv="+selected+"&stid="+nomer, false)
        req.send()
        if (req.status != 200) {
        alert( req.status + ': ' + req.statusText )
        } else {
        res = req.responseText
        alert(res)
        }
        if(res != "ERR")
        window.open(res, '_blank');
        else
        alert("Ошибка!")
    }
  }
}
</script>