Most car chargers are standalone devices in a parking lot. There is no teller to pay. I have never seen one with an attendant.
They could take cash automatically like a vending machine but that's going to require new staff to go around and collect money and deposit it. Probably much more expensive than taking credit cards.
Have the dispenser display a code for a $2 future credit. Or to be redeemed for cash at a later time somewhere cash transactions with humans exist. Or just use a standard vending machine module and give change with a restocking employee.
The market will decide if it's more expensive. Customers exist who will commute to another charger so they can pay cash. If the money left on the table is more than the cost of the cash module and attendant, it will be adopted.
They could take cash automatically like a vending machine but that's going to require new staff to go around and collect money and deposit it. Probably much more expensive than taking credit cards.