I do not have detailed knowledge about the various national postal services you mention, but I suspect you also don't? The German post uses custom EVs[1].
If you really want to disagree please find a source that says a service uses off the shelf vehicles.
Sorry, made an outdated assumption about Germany in particular.
The French post office, a pioneer in EVs (they've used them on and off since the early 20th century) and until recently the biggest postal user of EVs, uses off the shelf vehicles like the Renault Kangoo: https://www.lemonde.fr/economie/article/2019/04/08/la-poste-...
I don't mean to say that delivery companies must only use custom-built vehicles - but that basically all services have some custom vehicles. As an example the Royal Mail does seem to use a lot of production vehicles, but they also are working with Arrival for what look like custom made delivery vans. You can compare the Royal Mail vehicle made by Arrival[1] and their generic landing page for their van sales[2].
If you really want to disagree please find a source that says a service uses off the shelf vehicles.
[1] https://www.greencarreports.com/news/1103326_germans-to-buil...