On the confirm page, open the Server behaviors panel, Window -> Server behaviors.
in the list of server behaviors, locate the Local Checkout (Paypal Direct Payments) server behavior, and select it. Switch Dreamweaver to Code view. The code for the local checkout will be highlighted. You need to add an if statement around the opening and closing PHP tags of the highlighted code:
<?php if($<cartName>->GrandTotal() > 0) { ?>
<?php
...highlighted code...
?>
<?php }
else if (($_SERVER["REQUEST_METHOD"] == "POST") && (isset($_SERVER["HTTP_REFERER"]) && strpos(urldecode($_SERVER["HTTP_REFERER"]), urldecode($_SERVER["SERVER_NAME"].$_SERVER["PHP_SELF"])) > 0) && isset($_POST)) {
$<cartName>->redirStr = "checkout_success.php";
} ?>
Where <cartName> is the name of your eCart object.