Mayonnaise

Nobody is cheaper today. The best price per litre is €1.98/l and more than one supermarket charges it, so this shelf is a draw. Prices are re-checked daily and the answer changes with them.

Because a big pack and a small one are not the same purchase. Dividing each price by its pack size puts every option on one scale, so the cheapest ticket and the best value stop being the same question. Only offers priced per litre are ranked together; the rest are listed separately rather than mixed in.

Most supermarket items are own-brand, so the honest comparison is the shelf: every option in a category, ranked by what it costs per unit, each row naming its shop. Branded items sold in more than one supermarket still get a single page comparing them directly.