They have a monopoly on First Class letters, and still lose money.
They have the same problem that all government departments have.
The goal of managers in the public sector is to maximize power, get a greater staff, build perks. That means they need to expand their budgets. They need to hire more people. Efficiency is not a big concern. They want to have the best running department. Best being defined as the department that completes all tasks with no complaints.
When I was studying for a Master's in Urban Administration, I caused a bit of an uproar. I said the goal of a public administrator is to find a way to eliminate their own job. A great administrator would be seeking a permanent fix to a problem, not managing the problem well.
Is it out goal to have a very well managed Welfare state, or would it be better to have no need for a Welfare state?
The goal of private companies is to make money, so efficiency is key. They want to do the most with the least. They don't want to expand payroll, unless their is a need for more employees. And if the need falls, so do the number of employees. They want to maximize profits, and thus maximize their compensation.