whmcsTools/accept_free_order.php

<?php

// this hook sets the order status to accepted if order is free

if (!defined("WHMCS"))
    die("This file cannot be accessed directly");

add_hook('AfterShoppingCartCheckout', 1, function($vars) {
	if($vars['TotalDue'] == 0) {
	    $api = localAPI('acceptorder', array('orderid' => $vars['OrderID']));
	    if ($api['result'] == 'success') {
	        return true;
	    } else {
	        $errormessage .= $api['message'];
	        return false;
	    }
	}
});