at line 596:
<form name="checkout_PP_LI_Hid" action="https://www.PayPal.com/cgi-bin/webscr" method="post">
change that to:
<?php if($eCart1->GrandTotal() > 0) { ?>
<form name="checkout_PP_LI_Hid" action="https://www.PayPal.com/cgi-bin/webscr" method="post">
<?php } else { ?>
<form name="checkout_PP_LI_Hid" action="checkout_success.php" method="post">
<?php } ?>