Does anyone know if all the time, it is not possible for the staff to know if you tipped or not when using the terminal? Not for post-pay but pre-pay, e.g. buying a coffee. I would generally be inclined not to tip in these situations but am worried my drink will be contaminated if I don't.

I have a feeling the answer is yes, or don't know, and if there were any regulation enacted the best would be to ensure gratuities before service provided are either banned or guaranteed to be hidden. Gratuities after service provided don't seem to require any restriction.

Disclaimer: I live in a country without tips and love it, would feel better on trips to the US if it were the same.