A sticky add-to-cart button on mobile is the highest-leverage change most Shopify product pages never make. Here is the physics of the problem: your product page is long, because it has to be. Photos, variants, description, reviews, shipping details. The shopper scrolls down to get convinced, and the moment they are convinced, the buy button is three screens up. You built the desire and then hid the door.
On desktop nobody cares, the mouse travels for free. On a phone, every scroll is a thumb stroke, and the shopper who has to swipe back up past your size chart to find the button is a shopper you are actively testing for loyalty. Some pass the test. You are running a store, not a scavenger hunt, so stop administering it.
Where the thumb actually lives
Same product page, mid-scroll, decision made. The dashed arc is comfortable one-handed thumb reach.
The bottom edge of the viewport is the most reachable real estate on the phone. Rent it to your only button that makes money.
How to do it well
- Show price and variant in the bar. A sticky bar that says “Add to cart” next to the selected size and price is a mini order summary. One that just says “Buy” makes the shopper scroll up anyway to double-check.
- Appear on scroll, not on load. Let the bar slide in once the original button leaves the viewport. Two buy buttons on one screen looks desperate.
- Handle unselected variants gracefully. Tapping the bar with no size chosen should open the variant picker, not throw an error toast.
- Keep it thin. The bar is a courtesy, not a billboard. If it eats a quarter of the viewport, you have replaced one problem with another.
Many modern Shopify themes ship this as a toggle, buried under product page settings as “sticky add to cart” or “sticky buy bar”. Check your theme editor before you install anything. If your theme truly lacks it, it is a small snippet for any theme developer, and one of the rare cases where a single-purpose app earns its keep.
Then verify it on your own phone, one-handed, standing up. If your thumb can ride the whole page and buy without a single upward swipe, you are done. Tomorrow we will look at why mobile deserves this much attention in the first place, and what your analytics are hiding about it.
