If I were in your shoes, I'd test a sticky add-to-cart button on mobile this week, especially if my product pages require any amount of scrolling. The whole point of a good product page is to convince a customer to click "add to cart," as Nkechi Esan mentioned on an episode of Ecommerce Coffee Break. You never want that button to be out of reach when the motivation to buy strikes. On mobile, where screens are small and scrolling is constant, a sticky button keeps your most important call to action in view at all times. It's a simple way to reduce friction.
In the first week, I'd get it live. Many modern Shopify themes have this feature built-in, so I'd check my theme settings first. If not, there are plenty of solid apps for it. The immediate focus would be on the design. I’d take the advice Matthew Stafford gave on Firing The Man and make the button a color that stands out from everything else on the page. Brandon Landgraff made a great point on Ecommerce Coffee Break that this color should be consistent through the entire buying journey, from this button to the final checkout click. I'd keep the button itself simple: just "Add to Cart." Claus Lauter often emphasizes having a single, clear call to action on the product page, and I think that's critical here. No extra payment buttons or clutter.
Over the first month, I'd validate the change with data. You can't just assume it's working. This is a perfect candidate for A/B testing, which lets you prove whether the sticky button is actually increasing your conversion rate. Once I had some positive data, I'd experiment with adding a tiny bit of social proof near the button, a hack that John Weberg talked about on Firing The Man. It could be a simple star rating or a small icon, nothing that clutters the primary action.
The biggest thing I'd ignore is the temptation to add more features or text to the sticky element. Its job is to be a clean, simple, and unmissable target for a user who has decided to buy. Don't complicate it.
And the single biggest trap to avoid is 'set it and forget it.' You have to watch your analytics and, ideally, run a split test. Just because a feature is popular doesn't guarantee it will work for your specific audience or product. As Claus Lauter explained when talking about cart optimization, his older audience actually converted better with a traditional cart page than a pop-up cart drawer. The sticky button is the first step in that user flow, so you have to test and understand how it impacts the entire journey for your customers.
