Yep, done it good few times. My DS is 20 months and HATES the process of getting into the car seat. Once he is strapped in, he is as good as gold.
So this afternoon for instance, driving back from an adventure playground. He was exhausted. I swooped into a garage station, dashed into the shop, picked up milk, veg and juice, checked the car twice, paid, and then swooped back into the car. DS looked like he hadn't even clocked I had gone. He had been gazing out the window at the truck that had pulled into the garage.
If I had taken him out of the car, a 4/5 min dash, would have turned into a fifteen minute meander, no doubt a tantrumn thrown in as he was tired and hungry, and then a struggle to get him back into the car.
I absolutely do do it, and I will do it again. I would only do it if I can see the car from the shop.
So you can guess my answer to your question.... NO! You are NOT a terrible mother at all, and don't let anyone tell you otherwise.