Stripe configuration for closed event.

stripe.html

<script>

  var handler = StripeCheckout.configure({
    key: 'pk_your_public_key',
    token: function(token, args) {
      // Use the token to create the charge with a server-side script.
    },
    closed: function(e) {
      // The popup was closed.
    }
  });
  
</script>