Why does the app let an invalid quantity into the cart but block checkout?

Shoppers naturally pass through intermediate quantities while building a cart (for example, increasing toward a minimum or a pack multiple). To keep cart updates smooth:

• Above-maximum quantities are blocked immediately during cart interactions.
• Below-minimum and invalid-multiple quantities are allowed in the cart, then blocked when checkout starts — the shopper must correct the cart before the order can be placed.

This means the cart can temporarily show a quantity that checkout will not accept. The final Shopify Function validation is always the source of truth.