If merchant decides to provide their own custom checkout button (hideButton: true) to work with the Hosted Payment Page loaded by fatzebra.js, they will need to call the purchase method in order to trigger the payment flow. This method sends a trigger to the Hosted Payment Page within iframe's content view.

const fz = new FatZebra({
  username: 'xxx',
  test: true
}); 

fz.renderPaymentsPage({...}) // omitted

const handler = function() {
  fz.checkout();
}

document.getElementById('checkoutButton').addEventListener('click', handler);