One thing I find that might have been the problem is that the session is different in the middle of the confirm_cart.php page than where it's set for the errors at the top.
I see it as $_SESSION['WAVT_confirmcart_Errors']
Then on the confirm_cart.php page it is $_SESSION['WAVT_confirmphpcart_Errors']
There is an extra "php" in the middle of the reference to the session.
I removed that and it might be working now. Seems so anyway. I've only check this on my local server for that page. I didn't want to change the live site in case you still need to look at it as is.