modal.html

<form action-xhr="https://AmpedUpKids.com/OrderTrike" method="post">
  <fieldset>
    <!-- form inputs here -->
  </fieldset>
  <div submit-success>
    <template id="payTemplate" type="amp-mustache">
      <!-- checkout here -->
    </template>
  </div>
  <div submit-error>
    <template type="amp-mustache">
      <input type="checkbox" id="modal-hider">
      <div class="full-screen modal error hide-on-tap">
        <div class="panel">
          <h1 class="modal-head">{{error.heading}}</h1>
          <p>{{error.message}}</p>
          <button class="modal-btn"> OK </button>
        </div>
      </div>
      <label for="modal-hider" class="full-screen"></label>
    </template>
  </div>
</form>