Upsell App by Essentialwolf

Found this app to add a lastminute upsell before the Christmas sales push. Super quick to install, worked right out of the box (user error notwithstanding), support chat was incredibly quick to get back to me & were super helpful.

The only thing I can see missing is being able to customize the background colour of the slide badge on the slider widget, but there's lots of built in options and everything else is completely customizable so we're very happy overall.

very good app and free also good customer service

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.