Thanks for getting back to me.
I tried adding the session code (<?php echo $_SESSION['ProductCateogryID'] = $_GET['ProductCategoryID']; ?>) on the last line before the <html> tag in the results.php page and then updated the link on the details.php page but it did not work.
Looking at the site everything is fine up to the results page. Defaulting to rentals comes when I click on the learn more button on the results.php page and go to the details page.
I tried to change the link on the results.php page to the one you specified above, but that did not work either.
I am not sure if there is another page to modify at this point.