There are only two banks in the island National Bank of Greece (Chora and Potamos) and Piraeus Bank (at Chora). 1.5 km away from Aronis.
You can use in many places credit cards in the island. As is the case in most of Greece, American Express and Diner's won't work in most machines.
The public transport is minimal to non existent, the distances are long for an island. You would use a taxi, or rent a car if you do not have a