The penalties aren't that small, even for small amounts.
My father-in-law, who is one of those thousands of retired Minnesotans who spends the winter in the Harlingen, Texas area and does most of his shopping in Mexico, reported recently that one guy from Illinois who picked up just a couple of cases for some of his kinfolk was caught at the border and fined $10,000. and had his fancy motor home confiscated. My father-in-law had ceased to be willing to keep supplying his family several years before that because the agents were circulating the word that the crackdown was coming.
Some of the stuff being sold south of the border doesn't come anywhere near meeting automobile specs, either -- in some cases it's almost straight propane.