No sticky add-to-cart on mobile product pages
Observed: On a 390 px viewport the add-to-cart control scrolls out of view after the first gallery image and never returns. A shopper reading your description has no way to buy without scrolling back up.
Fix: Add a sticky bar that appears once the buy box leaves the viewport, carrying price, selected variant and the add-to-cart button. Minimum 44 px tap target.