I think that the problem is still that you are not using the correct value in the filtering. Rite now on the confirm page you have this code:
line 61
$colname_rsCounties = "-1";
if (isset($_GET['countyid'])) {
$colname_rsCounties = $_POST['countyid'];
}
You are still referencing the get version of this value so your SQL variable is not being set correctly and not setting the session variable accordingly. With a tax rule like you have so long as the session variable has a value it will trigger the rule.
You should also make sure to add a new hidden form element to the form on this page. In the new hidden form element you will set the name to countyid and for the value use the session variable. This will ensure that when you submit the order after confirming it you will still have a working charge.