Singletons, Enums and anonymous inner classes

As you may know, some people are declaring singletons with an Enum of 1 instance, because the JVM guarantees that there will always be a single instance with no concurrency problems to handle...

Thus what about an Enum with multiple instances? Can we say something like an Enum is a kind of ordered set of singletons sharing a common interface? Why?

public enum EnumPriceType {

    WITH_TAXES {
        @Override
        public float getPrice(float input) {
            return input*1.20f;
        }
        public String getFormattedPrice(float input) {
            return input*1.20f + " €";
        }
        },

    WITHOUT_TAXES {
        @Override
        public float getPrice(float input) {
            return input;
        }
    },
    ;

    public abstract float getPrice(float input);

    public static void main(String[] args) {
        WITH_TAXES.getFormattedPrice(33f);
    }

}

In this code why this doesn't work: WITH_TAXES.getFormattedPrice(33f); What is the interest of declaring a public method if it can't be called without passing through the common interface? I guess this is why i don't see any syntax to be able to declare an interface just for one of the instances of an Enum.

Edit:

It seems that enum instances are a special kind of anonymous classes. Thus i understand why you can't call that method.

My question is kinda related to: why can't an anonymous class implement an interface (in addition to the interface it may already implement!)

I totally understand why we CANT do that:

Vehicle veh = new Vehicle() {
    public String getName() {
        return "toto";
    }
};
veh.getName();

(getName here is not an override)

Why i don't understand is why we can't do that with anonymous classes:

Runnable veh = new Vehicle() implements Runnable {
    @Override
    public void run() {