I'm Richard - CRO Expert at PageFly Landing Page Builder. Welcome to Shopify Community!
Regarding your concern, please give me the URL of the page so I can check further for you.
In the meantime, if you'd like to achieve similar results, you could refer to the apps Reconvert Upsell and Cross sell or Cross Sell & Cart Upsell, in which Cross Sell app is already integrated with PageFly that allows you to easily use with drag-and-drop system after configuring the options from Cross Sell, you can check this manual for more details.
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Best of luck!
Not sure about that exact app but you can try out HypeCart for a similar experience - it's great for upsells and cross sells both in the cart and after the checkout.
You can find it here: https://apps.shopify.com/hypecart