Merchants using API version 63 and above are required to send an API request to the Mastercard Gateway using the INITIATE CHECKOUT apiOperation. This operation facilitates the initial step of processing a customer’s payment by creating a checkout session. Merchants can refer here to check their integration and make any necessary changes. 

Merchants using API version 62 and below are required to create a Hosted Checkout session using the CREATE_CHECKOUT_SESSION apiOperation. This operation initiates the payment process by generating a checkout session, which is essential for managing customer transactions securely. Merchants should refer here to check their integration and make any necessary changes. 

The steps detailed for either API version are essential for initiating the payment process and ensuring a smooth transaction flow. It is important to ensure all required parameters are correctly set in the API request to prevent errors or delays in processing. Additionally, merchants should verify that their system is properly configured to manage responses from the Mastercard Gateway, including transaction status updates and error messages. By adhering to these guidelines, merchants can offer an efficient checkout experience for their customers.