Yes Sir. It has been double checked by me before.
I've done an updating of both php files as described.
paypal_ipn.php - add "'s'." before substr($cart_PayPal_IPN_ID, strpos($cart_PayPal_IPN_ID, '-')+1);
ipn.php - replace all entries $_POST['invoice'] with substr($_POST['invoice'],1)
I can attach my files here if it will helps.