In a simple word, if your application is the only thing you have, you should go through the Google payment service, but if you sell your service through several environments (desktop / ios / windows mobile), and Android is one of them, you’re in okay.
Example; CBS application where, as soon as you pay, you can watch shows on desktop computers, on Android or on the iOS platform and use third-party payment processors.
But if you sell your application through a third-party payment service, this is not normal. For example, you have developed a chess game application that you can download for free on the Play Store. But after the download user has to pay and you collect the payment using a third-party payment service such as Paypal. The same applies to any other application features or digital content.
In one place I worked, where my company sold the application to the end user. We have the same application for Android, Desktop and Windows mobile. When a user pays for an Android app, they don’t get the free version of Desktop or the version for Windows Mobile. There were separate licensing agreements for each application, and the user must pay for each license separately. I remember that in this case, the lawyer of our company invited us to distribute our application outside the Play Store.
But finally, I recommend you contact a legal expert who understands the digital content policy. Better safe, then sorry.
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