Paying for an App which is Free

Hi

If I wanted to release an app on the Play Store for free but wanted the option that anyone can pay if they wanted to, how would I go about doing it? I dont want to have two apps, just one paid and one free.

Is it possible to have something similar to in app purchase but thats a way of thanking the dev? And does Google take a percentage from it? Or maybe PayPal link in the app?

Or any other methods?