I'm really not sure what the protocol is here. If I can park close to the building and see there's no queue, I leave my 2YO in his car seat in the petrol station forecourt. He's always been fine and so far I've never come back to a panicking child (touch wood). If the petrol station is busy then I usually take him in with me, which is a pain as a) it takes much longer and b) he wants to pick up everything in the shop.
What do you do?