Terminology:
Payment session: As soon as the user selects a supported coin on the payment page, we calculate the requested amount in the selected coin and create a unique URL (QR code) for the customer to scan or open in their favorite wallet.
The session is valid for 120 seconds and can be discarded manually by the customer. The moment we detect the expected funds, we mark the invoice as PAID and inform the merchant.
You do not necessarily need to access this API in your application unless you need to provide an in-app payment experience to your customers.
The easier way to go about this is to redirect the user to the payment URL obtained as a result of creating an invoice. Please see the Invoices page for details.