200 miles is well in range of most mid range EVs at this point. Vans exist on the market that can do that already. It's just a matter of putting in a bigger battery. Most companies buying vans buy the range they need rather than giving into range anxiety. There are loads of cheap options with limited range that are simply good enough.
Anyway, if that's more than even 1% of the market, I'd be very surprised. It certainly does not justify spending half a billion on inventing a vehicle with an option to have ICE capability (and the inherent design compromises that come with that) just as this market is being eaten up by basically most manufacturers launching specially designed EV only transport solutions.
A simple solution for this problem would be to simply keep on driving ICE vans on these longer routes until technology catches up. No need to sink money into R&D for this. It's completely redundant.
Replacing battery packs is just not a thing with EVs. Certainly not every year. There are enough first generation EVs still on their first battery pack to prove this. Besides, most vendors provide pretty nice warranties on their batteries because they know that they will rarely have to do any battery replacements. This is not a wait and we'll see kind of thing; companies buying these vehicles in large volumes tend to do their homework when it comes to cost.
All of this is not really something that needs researching, study, etc. US Post is technically kind of late to this. Besides, there are loads of electrical delivery vans in service across most delivery companies. Most of those are pretty much all switching as fast as they can to get some nice cost savings. The business case is long past needing a more evidence or proof. As a consequence, electrical vans are selling as fast as they can be built.