PaymentRequest.shippingOption

The shippingOption read-only attribute of the PaymentRequest interface returns either the id of a selected shipping option, null (if no shipping option was set to be selected) or a shipping option selected by the user. It is initially null by when no "selected" shipping options are provided.

This attribute is only populated if the constructor is called with the requestShipping flag set to true. If requestShipping was false (or missing),  shippingOption returns null, even the developer provides a  selected a shipping option.

// Returns the id of the selected PaymentShippingOption
var shippingOption = request.shippingOption; 

In the example below, the PaymentRequest.onshippingoptionchange and the PaymentRequest.onshippingaoptionchange events are dispatched. In each calls to updateDetails()  are made, one using a promise, and the other with a plain JS object. This demotrates synchrounous and asynchronous updates to a payment sheet. 

const request = new PaymentRequest(methodData, details, options);
// Async update to details
request.onshippingaddresschange = ev => {
  ev.updateWith(checkShipping(request));
};
// Sync update to the total
request.onshippingoptionchange = ev => {
  const shippingOption = shippingOptions.find(
    option => option.id === request.id
  );
  const newTotal = {
    currency: "USD",
    label: "Total due",
    value: calculateNewTotal(shippingOption),
  };
  ev.updateWith({ ...details, total: newTotal });
};
async function checkShipping(request) {
  try {
    const json = request.shippingAddress.toJSON();

    await ensureCanShipTo(json);
    const { shippingOptions, total } = await calculateShipping(json);

    return { ...details, shippingOptions, total };
  } catch (err) {
    return { ...details, error: `Sorry! we can't ship to your address.` };
  }
}