Enable a SSL certificate only for the membership checkout page

If you want to enable a SSL certificate for the membership subscription page, but do not want to use the Force SSL option from your Joomla! Global Configuration Settings, then you could try using a template overwrite for this functionality. The overwrite needs to be created for the components/com_rsmembership/views/subscribe/tmpl/default.php file and in the overwrite you will need to add:

<?php if($_SERVER['SERVER_PORT'] != '443') {

$url = "https://".$_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI']; 
$app = Joomla\CMS\Factory::getApplication();
$app->redirect($url);
exit();
}
?>

...at the beginning of the file.