What do you mean by "payment function"? Is it a one time purchase? Is it a subscription? If it's a subscription, then you must check that the user has a valid subscription and how do you do that? Is it at the request level in your application? If so, you know you can't hit the API of whichever server you're using for every request because the latency will be really, really bad. What will you do, then? Save the data into your database? If so, you'll have to think about syncing data between the service provider and your database.
It depends on what you want to do and your use-case and requirements.
It depends on what you want to do and your use-case and requirements.