Life of a payment transaction

Life of a payment transaction

Learn how merchants integrate payment apps and how payment transactions work
with the Payment Request API.

Appears in: Web Payments


Shipping and address support in the Payment Request API is removed from the specification and is no longer functional.

Web Payments APIs are dedicated payment features built into the browser for the first time. With Web Payments, merchant integration with payment apps becomes simpler while the customer experience gets streamlined and more secure.

To learn more about the benefits of using Web Payments check out Empowering payment apps with Web Payments.

This article walks you through a payment transaction on a merchant website and helps you understand how payment app integration works.

The process involves 6 steps:

  1. The merchant initiates a payment transaction.

  2. The merchant shows a payment button.

  3. The customer presses the payment button.

    A diagram of a cheese shop website with a BobPay (payment app) button.
  4. The browser launches the payment app.

    A diagram of the cheese shop website with BobPay app launched in a modal. The modal shows shipping options and total cost.
  5. If the customer changes payment method, the merchant updates the transaction details reflecting the change.

  6. After the customer confirms the purchase, the merchant validates the payment and completes the transaction.

    A diagram showing the customer pressing the "Pay" button in BobPay, followed by a diagram of the cheese shop page showing "Payment accepted".

Step 1: The merchant initiates a payment transaction #

When a customer decides to make a purchase, the merchant initiates the payment transaction by constructing a PaymentRequest object. This object includes important information about the transaction:

  • Acceptable payment methods and their data to process the transaction.
  • Details, such as the total price (required) and information about the items.
const request = new PaymentRequest([{
supportedMethods: '',
data: {
transactionId: '****'
}], {
displayItems: [{
label: 'Anvil L/S Crew Neck - Grey M x1',
amount: { currency: 'USD', value: '22.15' }
total: {
label: 'Total due',
amount: { currency: 'USD', value : '22.15' }

Including a transaction ID

Some payment handlers may require the merchant to provide the transaction ID which they have issued in advance as part of the transaction information. A typical integration includes communication between the merchant's and the payment handler's server to reserve the total price. This prevents malicious customers from manipulating the price and cheating the merchant with a validation at the end of the transaction.

The merchant can pass a transaction ID as part of the PaymentMethodData object's data property.

Provided the transaction information, the browser goes through a discovery process of payment apps specified in the PaymentRequest based on the payment method identifiers. This way, the browser can determine the payment app to launch as soon as the merchant is ready to proceed with the transaction.

To learn how the discovery process works in detail check out Setting up a payment method.

Step 2: The merchant shows a payment button #

Merchants can support many payment methods, but should only present the payment buttons for those that a customer can actually use. Showing a payment button that is unusable is poor user experience. If a merchant can predict that a payment method specified in the PaymentRequest object won't work for the customer, they can provide a fallback solution or not show that button at all.

Using a PaymentRequest instance, a merchant can query whether a customer has the payment app available.

Does the customer have the payment app available? #

The canMakePayment() method of PaymentRequest returns true if a payment app is available on the customer's device. "Available" means that a payment app that supports the payment method is discovered, and that the platform-specific payment app is installed, or the web-based payment app is ready to be registered.

const canMakePayment = await request.canMakePayment();
if (!canMakePayment) {
// Fallback to other means of payment or hide the button.

Step 3: The customer presses the payment button #

When the customer presses the payment button, the merchant calls the show() method of the PaymentRequest instance which immediately triggers the launch of the payment UI.

In case the final total price is set dynamically (for example, retrieved from a server), the merchant can defer the launch of the payment UI until the total is known.

Deferring the launch of the payment UI #

Check out a demo of deferring the payment UI until the final total price is determined.

To defer the payment UI, the merchant passes a promise to the show() method. The browser will show a loading indicator until the promise resolves and the transaction is ready to begin.

const getTotalAmount = async () => {
// Fetch the total amount from the server, etc.

try {
const result = await;
// Process the result…
} catch(e) {

If there is no promise specified as an argument for show(), the browser will launch the payment UI immediately.

If the payment handler is designed to return a transaction ID upon setting the total price, the ID can be passed as part of the promise result.

Step 4: The browser launches the payment app #

The browser can launch a platform-specific or a web-based payment app. (You can learn more about how Chrome determines which payment app to launch.)

How the payment app is built is up to the developer for the most part, but the events emitted from and to the merchant, as well as the structure of the data passed along with those events, are standardized.

When the payment app is launched, it receives the transaction information passed to the PaymentRequest object in Step 1, which includes the following:

  • Payment method data
  • Total price

The payment app uses the transaction information to label its UI.

Step 5: How a merchant can update the transaction details depending on customer's actions #

Customers have an option to change the transaction details such as payment method in the payment app. While the customer makes changes, the merchant receives the change events and updates the transaction details.

There are four types of events a merchant can receive:

  • Payment method change event
  • Merchant validation event

Payment method change event #

A payment app can support multiple payment methods and a merchant may offer a special discount depending on the customer's selection. To cover this use case, the payment method change event can inform the merchant of the new payment method so that they can update the total price with the discount and return it back to the payment app.

request.addEventListener('paymentmethodchange', e => {
// Add discount etc.

Merchant validation event #

For additional security, a payment app can perform a merchant validation before proceeding to the payment flow. The design of the validation mechanism is up to the payment app, but the merchant validation event serves to inform the merchant of the URL they can use to validate themselves.

request.addEventListener('merchantvalidation', e => {
// Use `e.validateURL` to validate

The support for the merchant validation event is limited to Apple Safari. Chromium-based browsers have not implemented this event as of May 2020.

Step 6: The merchant validates the payment and completes the transaction #

When the customer successfully authorizes the payment, the show() method returns a promise that resolves to a PaymentResponse. The PaymentResponse object includes the following information:

  • Payment result details

At this point, the browser UI may still show a loading indicator meaning that the transaction is not completed yet.

If the payment app is terminated because of a payment failure or error, the promise returned from show() rejects, and the browser terminates the payment transaction.

Processing and validating the payment #

The details in PaymentResponse is the payment credential object returned from the payment app. The merchant can use the credential to process or validate the payment. How this critical process works is up to the payment handler.

Completing or retrying the transaction #

After the merchant determines whether the transaction has succeded or failed, they can either:

  • Call the .complete() method to complete the transaction and dismiss the loading indicator.
  • Let the customer retry by calling the retry() method.
async function doPaymentRequest() {
try {
const request = new PaymentRequest(methodData, details);
const response = await;
await validateResponse(response);
} catch (err) {
// AbortError, SecurityError

async function validateResponse(response) {
try {
const errors = await checkAllValuesAreGood(response);
if (errors.length) {
await response.retry(errors);
return validateResponse(response);
await response.complete("success");
} catch (err) {
// Something went wrong…
await response.complete("fail");
// Must be called as a result of a click
// or some explicit user action.

Next Steps #

Last updated: Improve article