Click on "lawsuit" link.
The meat of filing is paragraphs 127-137.
Paragraphs 92-126 are also worth reading.
States have always been in charge of voting. Trump can't pass the SAVE Act. Knowing that a free and fair election will strip Trump's control of Congress, Trump is endeavoring to suffocate voters under the false pretense that mail-in voting is crooked. Four new USPS Board Trump appointees are all election deniers.
Link: https://www.cbsnews.com/news/states-sue-postal-service-mail-voting-rules/