Power to weight doesn’t matter as we are talking about using a solar panel instead of a roof. There’s no added weight. The car will already have inbuilt inverters so the only real weight add is the wiring. But also, this is a postal vehicle which will have large swings in weight anyways. A couple of extra pounds doesn’t make a difference here.
Further, this isn’t a car, which has a much smaller surface area. These things have about 10 square meters of flat roof. That’s a peak output of ~3kW. (realistically, probably closer to 1.5kW average throughout a day) which translates into 12kWh of charge in any given day. Roughly 10% of the battery capacity could be restored daily.
For large vehicles, like delivery vehicles and busses, the math on making the roof out solar panels instead of steel changes.