WASHINGTON — The US Postal Service’s promised all-electric fleet is still woefully behind schedule, with more than $3 billion in taxpayer funding out the door and just 612 of the expected 35,000 battery-powered delivery trucks built, according to a letter sent to Sen. Joni Ernst (R-Iowa) and obtained by The Post. Former President Joe Biden’s 2022 Inflation Reduction Act (IRA) set aside $3 billion for what Ernst has since ripped as a green “boondoggle” that saw almost all of that amount paid to Wisconsin-based defense contractor Oshkosh to build the environmentally friendly mail trucks. Ernst, who chairs the Senate’s DOGE...