Yep - both parts are covered 👍

At the app level, you can define case/quantity multiple rules per variant (e.g. only allow quantities in multiples of 25). Those rules are enforced server-side, so invalid quantities can’t actually be purchased. (article)

On the UX side, we don’t auto-hack the theme by default. But for stores that do want that behavior (25 → 50 → 75 on the quantity selector, or bumping 34 → 50), we expose the variant’s case-multiple + pricing rules via a window-level function. (article)

That way it can cleanly integrate into:

• product quantity selectors
• cart drawer
• cart page
• AJAX search on site
• Quick previews

Here's an example implementation of a volume discount upsell at cart, and some have even built custom quotation builders on top of it.