Are there APIs for app monetization?

Question

Mobile development

I want to write a mobile application, but I don’t know how to organize monetization... I don’t really want to make an additional server, because. I do one and I do not know how much it can stretch. Is there any (preferably free) cloud monetization solution? Those. which would mark, say, whether the user bought the application, in his database and actually implement the possibility of buying through the API.
P.S. I mean not just buying an application on Google Play, but so that it is initially free, and then you can buy additional functionality (permanently or temporarily, I haven’t decided yet).

Answer 1

Well, it seems like Apple has services for buying subscriptions and just paid things for this. Moreover, the rules prohibit the use of third-party services.
Google seemed to have the same