Ma'am, there is only one reason to leave your car running while you are at the bank, and that is if you are robbing the bank and wish to make a quick getaway.
If you are merely stopping at the ATM, turn off the engine of your convertible Mercedes coupe and spare the air for the rest of us. Clearly, you were not in such a hurry that you couldn't spare the 2 seconds it takes to turn the ignition back on, as you sat in your car for a couple of minutes and sifted through your purse before departing.
You aren't saving gas by leaving the car running -- any car with fuel injection, and certainly a fancy late-model Mercedes, doesn't waste gas when starting up. An idling car wastes a good deal of gas.
If you are merely wishing to flaunt your affluent indifference to the astronomical price of gas, why not do it in a more eco-friendly way? Turn off your car, take the money you just withdrew from the ATM and tear it up into tiny pieces. Then place the bits in a recycling bin.