Where? Germany? In France I don't know what is the regulation but still many places will ask for minimum charges, sometimes around 10€.
In the UK I sometimes pay for a single Banana by card, 50p (0.6€).
When I do, I wonder if the highest carbon footprint was from the payment processing or the banana processing!
Germany but if there's an actual law it's probably EU-wide. I haven't found one but I have found rumors that PSD2 banned minimum charges. I guess everyone is complying to a phantom law.
AFAIK they've been allowed and disallowed and re-allowed under contracts with the payment networks over the years; currently I think at least in the US they're allowed, but regardless, I'm not aware of them having been illegal.