Yeh the only problem being is that the supermarket chargers are generally the slowest chargers
With respect, you can’t have it all ways. If you want the cheap running costs you have to be prepared to make the sacrifice with your time. Why should a hard working business owner subsidise the motoring costs of members of the public? Put it another way, if you had a driveway with a charger, would you be happy if a random member of public parked their cars on your drive and charged their car at your expense? It’s no different from what you are proposing!