Actually, got that sorted, the limit “by product” made it sound as if by number of products, not the setting to use for any limit type, in my case weight.
So that’s working, and it stops someone from selecting more than the max on the product page. But they can edit their quantity on the cart to exceed the limit. Clicking checkout does keep them on cart with the message “You can only buy maximum 150 of my product.” but because my criteria is WEIGHT, the message doesn’t help a lot. I edited the error messages to explain it was the total weight, but the message still doesn’t change when it appears on site…