An Upsell App Killed My Add-to-Cart Button

I installed one of the top-rated upsell apps on the Shopify App Store. 1,200+ reviews, 4.7 stars. It was supposed to increase average order value by showing "frequently bought together" recommendations on the product page.

Instead, it broke the Add-to-Cart button. But only on Safari. And only on iOS 16 and below. Which is about 40% of my mobile traffic.

The Debugging Timeline

Day 1: Installed the app. Tested on my laptop (Chrome, Mac). Everything worked. Went live.

Day 2-3: Noticed a slight dip in conversion rate. Attributed it to normal fluctuation.

Day 4: Got an email from a customer: "Your Add to Cart button doesn't work on my iPad." I tested on my iPad — worked fine (I had iOS 17).

Day 5: Asked a friend with an older iPad to test. Add-to-Cart button completely unresponsive. No error message. Just... nothing happened when you tapped it.

Day 6: Found it. The upsell app used a JavaScript API (Array.findLast) that doesn't exist in Safari versions before iOS 17. The script crashed silently. Because it loaded before my theme's Add-to-Cart handler, the crash prevented the cart handler from ever registering.

Estimated loss: 40% mobile traffic × 5 days × ~$200/day = $400 in lost orders. From a top-rated app with 1,200 reviews.