Before the cart closes
The question that decides the sale gets answered.
Someone is on the product page with a card in their hand, wondering whether it will fit, whether it's really in stock, and whether it arrives before Saturday. They ask, and the answer comes from your catalogue and your shipping zones — in seconds, at the moment it still matters.
It only recommends what someone can actually buy today. No suggesting a size that sold out this morning, and no delivery date it made up.