The ones who in 2006 forced the USPS to pre-fund their retirement obligations.
https://about.usps.com/who-we-are/financials/annual-reports/...
>>Unlike any other public or private entity, under a 2006 law, the U.S. Postal Service must pre-fund retiree health benefits. We must pay today for benefits that will not be paid out until some future date. Other federal agencies and most private sector companies use a “pay-as-you-go” system, by which the entity pays premiums as they are billed. Shifting to such a system would equate to an average of $5.65 billion in additional cash flow per year through 2016, and save the Postal Service an estimated $50 billion over the next ten years. With the announcement of our Action Plan in March, we began laying the foundation for change, requesting that Congress restructure this obligation. >> >>The pre-funding requirement, as it currently stands, contributes significantly to postal losses. Under current law, the Postal Service must follow a mandated pre-funding schedule of $5.5 billion to $5.8 billion per year through 2016.
Just from this graph, it looks like since 1960, the only time they ran a surplus was 1994 - 2004: https://upload.wikimedia.org/wikipedia/commons/4/49/USPS_Sur...