The USPS wants one vehicle so it needs a much longer range. The Grumman LLV was specifically designed for the United States Postal Service with Grumman winning the contract for production... Approximately 140,000 LLVs are in the USPS delivery fleet.[6][7] A number were also sold to Canada, Mexico, and several other countries.

https://en.wikipedia.org/wiki/Grumman_LLV On September 22, 2016, the United States Postal Service awarded the NGDV Prototype Contract to six selected suppliers: AM General, Karsan, Mahindra, Oshkosh, Utilimaster, and VT Hackney. Half of the prototypes will feature hybrid and new technologies, including alternative fuel capabilities. The prototypes will represent a variety of vehicle sizes and drive configurations, in addition to advanced powertrains and a range of hybrid technologies.[16]

The old LLVs were finally retired in Canada a few years ago due to spontaneously catching fire[1] but lasted 28yrs of heavy service.

[1]http://postalnews.com/blog/category/llv-fires/

That's the kind of thinking that leads to the JSF/F-35. :)

Sometimes, but economies of scale are moreover noticeable when you make 100,000+ of something than 200+.

I would say go electric, but with N optional and easily replaceable battery packs. So, routs that need 50 mile range get that and you can scale to 100, 150 etc, or add an extra pack when range starts to drop etc. This also lets you keep a motor pool of being repaired without needing an extra of each model.