The #SumUp card payment terminals seem to be quite popular, also among communities taking donations, e.g. at #FrOSCon.
For @Teckids , I made a small stand-alone POS frontend, so we can take card payments and display QR codes for receipts to customers while avoiding the proprietary Google Play app and not passing on customers' e-mail addresses to SumUp.
It's very basic for now, but find it here:
