I'm developing an Android app, in two different versions: free and paid.
The free version is a stand-alone app, working on its own on the user device. Users can store their data and access it within the app.
The paid version can also connect to a server and synchronize the user data with an external server. The user can also access its data on the server itself via another website, totally unrelated to the Android app.
Do I have to use Google Play's in-app billing service to unlock the paid version, if the website and the server are not dependent on the app and can be accessed from a desktop browser?
What the paid version is offering is a synchronization mechanism between the app itself and the website.
To clarify even further:
Users can consume their own digital contents (their data) outside the app (i.e. on the website) and theoretically even ignore the app completely and still have the full service (they can generate such data on the website and consume it on the website). By ignoring the app, what they are missing is just the availability of their own data on the device.
My app will not redirect the user to an external payment processor, the payment will be made solely from the website.
Developers offering products within a game downloaded on Google Play or providing access to game content must use Google Play In-app Billing as the method of payment.
Developers offering products within another category of app downloaded on Google Play must use Google Play In-app Billing as the method of payment, except for the following cases:
So, in your case, the answer is probably that you must use Google billing.
https://play.google.com/intl/en/about/monetization-ads/
Using external payments processor on Android for SaaS instead of Google Play In-app Billing