OK, on line 3 of the success page you are already passing the email address forward:
die('<script>document.location.href="http://www.bandofhonor.com/checkout_success.php?email='.$_POST['x_email'].'";</script>');
To get the other fields from authorize.net you would have to pass them forward as well like:
die('<script>document.location.href="http://www.bandofhonor.com/checkout_success.php?email='.$_POST['x_email'].'&firstname='.$_POST['x_first_name'].'';</script>');
Then you could refer to the first name in your email with <?php echo($_GET['firstname']); ?>
For a complete list of variables passed back to your page see: http://www.authorize.net/support/SIM_guide.pdf