Our mailwoman's action is a classic candidate for supererogation.
However, despite its optionality, the mailwoman's action was hardly a matter of moral indifference.
Although the mailwoman's optional rescue was not a matter of indifference, that she wore black socks that day was.
Let me briefly address a possible objection to the sufficiency of our definiens--one to the effect that it makes the supererogation operator susceptible to an analogue of "Ross's Paradox".(16) Recall the earlier example of the child rescued by the mailwoman from the fire.
In the context of the case described, if we were to say, "it was supererogatory for the mailwoman not to opt for running down the block, pulling the alarm, etc.", this would seem right; and so forth.(17) Secondly, we could take the unheroic way out and retreat from the proposed analysis by reading the definiendum more cautiously as "It is supererogatory to permissibly see to it that p" (to see to p while doing only permissible things).