But I still can't convince my mother that paying $10 for delivery of a $3 peg is better than driving all the way to a remote storehouse to get it yourself, even though the cost of the delivery is significantly greater than cost of the goods and hence feels "not worth it".

I agree that that is a common and buggy way to think. I probably tend to do that myself when I'm not careful, so I concede that it is a problem. But your example has someone comparing part of the cost of an action with another part of the cost of that same action. I'm talking about comparing the cost with the benefit of the same action. Are you saying that even doing that could put one at more risk of making the mistaken comparison that you describe?

At any rate, that wasn't the kind of comparison that lukeprog was doing. Could you elaborate on the kinds of mistakes that you see flowing from what he said? One possible confusion is using the sign of (benefit) – (cost) of an action (measured in lives saved, say) to decide whether to do it. Do you see others?