your code will create a session variable named "estimates_estimates_id":
$WA_sessionName = "estimates_estimates_id";
Try adding the following at line 1 of the success page:
<?php if(!session_id()) session_start(); ?>
if it is still not working, add the following after the body tag to display all sessions that are set:
<pre><?php var_dump($_SESSION); ?></pre>
check to see if the estimates_estimates_id variable is listed in the session dump.
If it is not, there may be a problem on the server. I am attaching a sessionTest.php file to this message, upload it to the server to test the servers session management abilities. If it fails the test, you will need to contact the host, if it passes, add your insert page and the success page to a zip file and send that with your reply.