I think what they’re implying is that it would not be feasible to seize or remove properties already owned by corporations, and by the time reforms are actually attempted they will own a significant portion of properties, making it extremely difficult to get those already-owned houses in the hands of first time buyers.
The first thing that comes to mind is that if they’re custom they can put the driver seat on the right side, which makes stopping at mail boxes much easier. I don’t know of any non-import vehicles that are street legal in the US outside the USPS. It makes sense at this scale as well. A quick search shows that one line of cars, the Nissan Altima, sold just shy of 60,000 vehicles in a year. I don’t know if that’s a good benchmark for sales needed to make designing a vehicle worthwhile, but there are already 60-80,000 new USPS trucks ordered, and at that point since the designers would be working with one organization instead of trying to market to thousands of consumers it’s probably easy enough to build a custom car for that organization’s needs.