Profit is the incentive. There should be no breaking even, or any other money making measures. It should be taxes that pay all operating costs, because the service is in the Publics best interest.
It's similar to having a Military, but required it to pay for itself. It could also be applied to the Police, they don't need to pay for themselves either. Making them need to break even, is stupidity of the highest order.
Capitalism captured our Post office, and it shouldn't have ever been in play in the first place.