this sounds like you have the SSL server on a different domain then the main shopping cart.
it gets difficult to clear the sessions in both places since there is rely not a way to display the success page with the order number in it on the SSL Site, and also trigger clearing the cart on the none ssl domain.
you could add a click here to continue back to the site link:
<a href="http://www.yourdomain.com/continue.php">click here to continue back to the site</a>
on the continue.php page add the code to clear the session, and add redirect if cart is empty behavior after it to direct to the main site index page.