The Payment Request API provides a consistent user experience for both merchants and users. It is not a new way for paying for things; rather, it's a way for users to select their preferred way of paying for things, and make that information available to a merchant.
Many problems related to online shopping-cart abandonment can be traced to checkout forms, which can be difficult and time consuming to fill out and often require multiple steps to complete. The Payment Request API is meant to reduce the number of steps needed to complete a payment online, potentially doing away with checkout forms. It aims to make the checkout process easier, by having payment apps store a user's details, which are then passed along to a merchant, hopefully without requiring an HTML form.
To request a payment, a web page creates a
PaymentRequest object in response to a user action that initiates a payment, such as clicking a "Purchase" button. The
PaymentRequest allows the web page to exchange information with the user agent while the user provides input to complete the transaction.
You can find a complete guide in Using the Payment Request API.