Buy Android app access here, then open the app with the same email address.
Buy access on the website, open the app from Google Play, and sign in with the same email address you used at checkout.
Complete your purchase on the website with Stripe using the payment methods available on your device and browser.
Your purchase is tied to the email you use here, so that same email should be used inside the Android app.
This purchase unlocks the Android app only. Website study access and the optional audio add-on are purchased separately.
If you change devices, just install the app again and sign in with the same email. You do not need to buy it a second time.