How do I add a sticky add-to-cart bar on mobile?

A sticky add-to-cart bar that appears once the main buy button scrolls out of view is a proven conversion lift on mobile, where product pages are long. Some themes include it as a setting — check first. If not, it's a small custom addition to the theme: a fixed bar showing the product, price, variant, and an add button, shown after the user scrolls past the hero.

Doing it in the theme (vs. an app) keeps it fast and on-brand with no extra scripts. Keep it unobtrusive, make sure it respects variant selection, and test it doesn't cover key content or the cart on small screens.