index.html

    <script type="module">
      import SimplePayment from "https://unpkg.com/payment-components@0.1.0/dist/payment-components.esm.production.js";

      async function checkout() {
        const response = await fetch("/pay", { method: "POST" });
        const data = await response.json();
        return { signedRequestData: data };
      }
      
      document.addEventListener("DOMContentLoaded", function() {
        const sp = new SimplePayment(checkout, {
          redirectUrl: window.location.href,
          networkId: 4
        });
        // fil in with the id of your own button
        sp.init("#checkout-button");
      });
    </script>