10 year procurement process and 10 year purchase windows tends to add a price premium.

That said, the rivian delivery van is in the neighborhood of $85k-$90k [1], so not that far off price wise, and the NGDV has a lot of design elements that make it more precisely matched for how the USPS uses delivery vehicles. The article I linksd does point to a much less expensive Ford E-Transit, but it's still not built around the USPS's needs.

[1] https://www.caranddriver.com/rivian/commercial-van