The $3B order was placed in March 2022 for a mix of 50K EV and conventionally powered vehicles, EVs being at least 10K. Delivery was supposed to start in October 2023, but that got pushed out to June 2024. It sounds like the mess-up is a mix of USPS incompetency, Oshkosh Defense not having a mature design, EPA envirocrats' posturing and meddling messing with the design and purchase mix, and the lack of charger and other relevant infrastructure.
When bureaucrats say, "Make it so," that does not mean they know it's possible. One goal that the prototypes seem to meet is that the things are so ugly that no sane or half-sane person would steal one.