We spent a lot of time trialling Android devices but ran into several issues that resulted in a negative user experience.
NFC chip located on the rear of the device
This means when users are 'tapping to pay' they must tap the back of your device. Most payment terminals in use today prompt users to tap on the front of the device. Despite graphical and text prompts instructing people to tap on the back of the device, most people just kept trying to tap on the front of the device until they gave up resulting in a lost tip!
NFC chip located on the rear of the device
We found the exact NFC chip location varied on Android from manufacturer to manufacturer and model to model. Some it was at the top center, others top right, others middle center. This resulted in users tapping all over the device unsuccessfully. To provide an acceptable user experience, much more development is required to identify and adequately instruct the user where to tap to be able to successfully place a tip.
Payment screen set by Stripe
We found that with Android, the payment screen is set by Stripe and cannot be changed. This means our ability to successfully instruct users where to tip is further limited when it's actually time for them to tap to tip.
As user experience is of utmost importance to Tipper's success we are considering the way forwards for Android users.
In the meantime we recommend you acquire a compatible iPhone (new or used) and start getting more tips!