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Very easy: offer them a long trial (numbers will show that your revenue is hurt, but don't care). Eventually the trail runs out, and they search how to renew or they ask support. The first result appears: your payment subscription page.

If a customer has used your product already for long time 20 days+, extra steps to set up payment one time, especially when you do not auto charge short trials, the customer is happy.

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so you offer a free trial on google play that is long enough to get people hooked. but there is no paid app on google play. instead people have to pay through other means to unlock the app?
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Yeah I need more details too. Does Google play trial not auto charge by after the trial ends?
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i have no clue what features google play has but my guess is that a simple date check that disables the app after a certain time is possible without using any google play features. so it could be offered as a free app that simply stops working after some time.
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They push the user to their website where subscriptions are.
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