With Payments standard, on the confirm page, the form posts to the paypal server:
php:
<form name="checkout_PP_LI_Hid" action="https://www.PayPal.com/cgi-bin/webscr" method="post">
you need to write an it statement to post to the checkout_success page if the amount in the cart is 0:
php:
<?php if($<cartName>->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 } ?>
where <cartName> is the name of your eCart object.
