Oh that seems difficult. In Finland the postal service uses right hand drive cars.

They do in the US too, but contractors use their own vehicles. Imported right-hand drive cars can be imported, but their use is rare.

This is partially because the car dealer lobby convinced congress to prevent the importation of cars less than 25 years old. Ironically, they did so using safety regulations, so a brand-new car meeting current European safety regulations is legally unsafe, but a 25-year-old car, complying with what European safety regulations were 25 years ago, less any deterioration in those safety systems, is legally safe.

This is false.

Most rural routes are served by the USPS-proper.

If most, but not all routes are served by USPS proper, and the rest are contracted out, than it is true the USPS contracts out routes.

> in many states it's legal for the contractor to drive from the passenger seat.

I doubt they're driving from the passenger seat. It's hard to reach the pedals from the passenger seat. Many mail delivery vehicles in the US are right hand drive; the driver's seat is on the right. I don't know that left hand drive is strictly required by any states, it's more a matter of there's not many models available for sale in the US with right hand drive as an option, and few people order them when available; and importing out of market RHD cars is a chore.

You can (or could) order some Jeeps in RHD, and a lot of rural USPS contractors have RHD minivans (many from the Japanese Domestic Market)

I did see a 'converted' RHD Windstar (Ford minivan) on FB Marketplace, where they removed the driver seat, put a flat platform in, and connected controls with cables and levers. That's an option too, I guess.

When I briefly lived in a rural small town, several years ago, the mail delivery contractor who served our house used exactly such a converted minivan.