You are getting this error because the variable you are referencing in your set session value is from the recordset and this code occurs before the recordset is created. To resolve this problem you will need to move the code at line 15 to be just after what is currently on line 80. It would be best to have it in it's own code block below this.
Move this after line 80:
<?php
if (!session_id()) session_start();
if($_SERVER["REQUEST_METHOD"] == "POST") {
$_SESSION["eCartCheckoutForm_counties"] = "".$row_rsCounties['tax'] ."";
}
?>