They could copy China's WeChat solution. Here, the buskers have a QR code where you can use to donate to them. You scan the code, and then you can select the a

They could copy China's WeChat solution. Here, the buskers have a QR code where you can use to donate to them. You scan the code, and then you can select the amount to donate. No external devices needed where you aren't sure about the safety of it and no need to be plugged in. Just a simple QR code.