The paymentManager property of the ServiceWorkerRegistration interface returns a payment app's PaymentManager instance, which is used to manage various payment app functionality.

A PaymentManager object instance.

navigator.serviceWorker.register("serviceworker.js").then((registration) => {
  registration.paymentManager.userHint = "Card number should be 16 digits";

  registration.paymentManager
    .enableDelegations(["shippingAddress", "payerName"])
    .then(() => {
      // ...
    });

  // ...
});

• PaymentManager.userHint is used to provide a hint for the browser to display along with the payment app's name and icon in the Payment Handler UI.
• PaymentManager.enableDelegations() is used to delegate responsibility for providing various parts of the required payment information to the payment app rather than collecting it from within the merchant website.

• Payment Handler API
• Web-based payment apps overview
• Setting up a payment method
• Life of a payment transaction
• Using the Payment Request API
• Payment processing concepts