Even a 75kWh battery would be expensive and heavy, and probably overkill in this case. If 100 miles of range is more than enough for most routes for the day, then something like 30kWh might be more reasonable.
Also one might be able to keep costs down by optimizing for cost rather than energy density. LiFeP04 cells have been comparatively expensive, but it seems price has been dropping lately. It's also a good choice for safety and durability reasons, and because they don't use cobalt which is sort of expensive and sort of rare. Not using cobalt means avoiding a resource that would bottleneck other manufacturers of EVs.
It'd be interesting to know what the actual specs are on the EV version of the USPS vans (battery type, capacity, etc...) and see if my speculation corresponds to what they chose. I don't know if they've released that publicly yet.
A 30kWh battery could easily be 5 or 10 thousand dollars, which may well be worth it to save on gas for a long time; on the other hand, gas engines are well-understood and really cheap, because we have a huge established industry that builds engines by the millions and has been optimizing for cost for over a hundred years.
This sounds a lot like the "Captain Samuel Vimes 'Boots' theory of socioeconomic unfairness" from Terry Pratchett that generally eventually gets quoted in this sorts of discussion.
https://www.goodreads.com/quotes/72745-the-reason-that-the-r...