openWindow(url)

self.addEventListener("paymentrequest", async (e) => {
  // …
  // Retain a promise for future resolution
  // Polyfill for PromiseResolver at link below.
  resolver = new PromiseResolver();

  // Pass a promise that resolves when payment is done.
  e.respondWith(resolver.promise);
  // Open the checkout page.
  try {
    // Open the window and preserve the client
    client = await e.openWindow(checkoutURL);
    if (!client) {
      // Reject if the window fails to open
      throw new Error("Failed to open window");
    }
  } catch (err) {
    // Reject the promise on failure
    resolver.reject(err);
  }
});