There aren't that many cars that it would get off the road and it's not on mission and outside the realm of their expertise, so no, it's not a good way for them to make money which they shouldn't be doing anyways.
This weather data wouldn't be as useful as you think it would be. You'd be getting data only taken once per day more or less at the same time. You're much better using the airstrip a mile away that has a sensor that takes readings every 10 minutes or whatever.
There are 190,000 mail trucks delivering mail every day. I’ll bet NOAA would find 190,000 mobile sensors collecting data eight hours a day, most within a very small radius, quite a compelling dataset. There aren’t that many airstrips. Not even close.
And who cares if it’s not on mission? We can change the mission.
That's a fair point, NOAA might have a use for it, I don't know.
However, selling data to Google still seems like a bad idea. Things that are off mission and the point is to make money are just a distraction with no good purpose that's going to make the post office be greedy instead of just focusing on delivering the mail.
I could maybe agree that there was some value in having a public set of streetview like images (which is the more likely thing they'd do, not sell it to Google or whomever which would just be terrible), but it also seems deeply creepy. Then again, Google is already doing it so I suppose there's an argument that at least this way it's public domain.
This weather data wouldn't be as useful as you think it would be. You'd be getting data only taken once per day more or less at the same time. You're much better using the airstrip a mile away that has a sensor that takes readings every 10 minutes or whatever.