USPS is self-funded through postage and other services they provide. They did receive $50 billion from an emergency funding bill back in '22, but that also removed a hefty, arbitrary load: back in 2006, legislation was passed that forced the USPS into pre-fund retirement plans for the next 75 years. The emergency funding bill lifted that requirement and gave emergency funding to the USPS under the condition that the USPS adopt changes listed in the bill to further address funding shortfalls and become solvent again.
The people paying for USPS windshield replacements are effectively paying customers, much like how toll roads are supposed to fund their own upkeep by charging for its use.