Results of "checkoutAdvertise" trigger are not within checkoutForm form, this limits our plugins to create custom form inputs for checkout page.
Can we move the output within a form of cart's default.php template:
<form method="post" id="checkoutForm" name="checkoutForm" action="<?php echo JRoute::_( 'index.php?option=com_virtuemart&view=cart'.$taskRoute,$this->useXHTML,$this->useSSL ); ?>">
<div id="checkout-advertise-box">
<?php
if (!empty($this->checkoutAdvertise)) {
foreach ($this->checkoutAdvertise as $checkoutAdvertise) {
?>
<div class="checkout-advertise">
<?php echo $checkoutAdvertise; ?>
</div>
<?php
}
}
?>
</div>