I'm just going to admit it. I have 3 kids, 5, 3 and 2. I take them on EVERY errand with me. There are some tiny pit stops we make where I leave them in the car: Under one minute, in full view of the car at all times, I lock it and take the keys with me, when I know the environment is completely crime free and safe, and the temperature is fine. The post office is one of these errands. I often have to drop a prepaid package etc. Our town is small, there is never a line, the car is in full view, there is no speeding traffic to hit the car in the lot which is separate from street. etc. Literally, a meteor would have to fall out of the sky onto the car in the 30 seconds it takes me to drop the package for anything to happen. I also go in to pay for a pump in our station where they have to manually run our grocery card points. Again, in full view of the car at all times, I wait until line is down to one person or less, my keys are not left in the car to jack it, etc. I also pick up dry cleaning-same scenario, car is in front of window, parking lot is removed from street, desk is 3 feet from window, keys are with me. I pick up pizza from the pizzeria, safe parking, full view of car, if order isn't ready I go back to car to wait. Etc. My kids are very obedient and have NEVER ignored the command: Do not unbuckle. They would NEVER climb into the front seat etc. If they did, I would see them, and react immediately.
There are those who say you can never ever ever leave a child in a car for one single second. Kudos to them. I think instinct can be relied upon though. Sometimes you simply can't unstrap 3 kids including one or two who are asleep to do something that takes 30 seconds on a whole day full of errands. Again, kudos to those who can.
Anything that could remotely take 3 minutes or more and would mean going into a building where I couldn't see the car. No. I wouldn't do that. And I grew up being left in the car so long on my mom's errands, I once cracked the windshield with my forehead while jumping up and down on the front seat :-0