post parameter ‘name’ 404 error

It sounds as though WorldPay is responsible for sending the overly generic name parameter so you aren’t in a position to control that. You will instead have to catch the request and force it to work. Something like:

function redir_404_wpse_137703() {
  if (is_404() && isset($_GET['name'])) {
    locate_template('some-theme-template.php',true);
    exit;
  }
}
add_action('template_redirect','redir_404_wpse_137703');

some-theme-template.php will need to be constructed with get_header(), content, and get_footer() like normal custom templates.

The big problem I see is sorting out genuine 404s from WorldPay ones but there must be other parameters that would help sort that out.