Lysander Spooner was an early anarchist who sought to make mail accessible to everyone, and started the American Letter Mail Company to challenge the government. He was wildly successful, and though the government drove him into the ground in court, his legacy is the now common public expectation that stamps should be affordable for everyone.

His system included free local delivery, but otherwise, our current postal system could operate with little adaptation to a solarpunk world. Mail is machine sorted, high-tech logistical systems save fuel and time while maximizing the amount of mail transported. Postal workers are all organized. Sending mail is available to everyone, not just an exclusive class.

I guess the biggest change would be workplace democracy, so that postal workers could elect their managers, and a situation where someone like Louis DeJoy being in charge would be impossible.