My parents used to do this with me when I was 4 or 5 I believe - at normal shops, not just paying for fuel. This was a different country though so I could usually see them in the shop as we were parked directly outside.
They would leave the engine on with the AC running, but I was sufficiently young dumb to be scared by the noise and I thought it meant the car might explode at any moment. I never thought to mention this to my parents though, so they honestly believed I was fine!! Just mentioning it as other kids might feel the same but not say so.
I've left DS (1yr) when paying for petrol but have rarely been longer than 5 min. Not sure I'd feel safe leaving him for longer TBH