I would like you to print out some of these values so that we can see what they are. In your confirm page just after line 87 add in this block of code:
<?php die("the value of the eCartCheckoutForm_counties variable is: " . $_SESSION["eCartCheckoutForm_counties"] . "<br/>The value of the posted county id is: " . $_POST['countyid'] . "<br/>The value of the tax column is: " . $row_rsCounties['tax']);
This will print out the values for the session variable, the posted county id, and the tax rate from the rs. This will help us determine where the problem is. Please post back with your results after you put this code in place and run a test.