Changes how the Postal Regulatory Commission (PRC) and the U.S. Postal Service (USPS) set and review market‑dominant rates, enforce unlawful rates and missed performance targets, and handle complaints and proposed service changes. It tightens complaint and review timelines, limits annual price growth (CPI‑U minus 0.5 percentage points by default), creates a PRC Office of the Customer Advocate, requires an independent demand model, and allows a portion of the Postal Service Retiree Health Benefits Fund to be invested in index funds under new oversight rules.