Upsell App by Essentialwolf

We had another upsell app installed but were nervous about the "views" metric for upsell products being tied to the monthly app cost involved - which essentially punishes us for high traffic without any sales. So we promptly ditched that app for the Essentialwolf Upsell app and couldn't be happier. Setup was a breeze, integrated seamlessly with our theme, and support has been very prompt.
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Update: upsell.essentialwolf.com refused to connect (HTTP 502 - Bad Gateway) — We have recently had 2 outages within 2 weeks (possibly more, since we were not monitoring for this until recently) — this means the widget doesn't appear and customers are unable to add required components to their order. We have no idea how many sales have been lost because of this. They gave us no information about what caused the outage or how long the service had been down. You'd think they'd have some form of server monitoring for this, so that store owners don't need to advise them when their app stops working. As a result of this, we're now looking at alternatives, which is a shame, because the app is good, but we cannot afford this sort of unreliability.

We’ve been using the Upsell App by Essentialwolf with the Symmetry theme, and it works perfectly. The integration is seamless, easy to set up, and runs smoothly without any issues. It has helped improve our upsell flow and overall store experience. Highly recommended for Shopify stores.

This app is absolutely worth the price, and works really well!

Whilst having not liaised wit their support team at all, so I cannot comment on that, I think not having to do so adds credibility to how simple this is to use.