What if the customer took the vegetable and put it in a bag which sits on a scale right in front of the shelf. Then they take the bag and go.

Alternatively, the entire shelf is a scale, but the shelves are smaller so that only one person can pick vegetables at a time.

Forcing the customer to drop the item(s) on a scale seems most reasonable, but requires relatively strong controls to ensure the customer isn't pulling shenanigans like placing item A and item B into a bag, then co-mingling them when weighing them later.