We have tried almost all of the major upsells apps for Shopify and Candy Cart is really the best one we have found if you cannot or do not want to implement a custom checkout gateway like OneClickUpsell. It's also way cheaper!

The popup is great looking and easy to use on both mobile and desktop, it's power is the ability to offer many types of upsells in one single popup. Consider installing this app now when the price is low as it will surely go up soon. The customer support is among the best we have experienced with a Shopify app, very fast responses.

It's almost the perfect upsell app for us and we have just requested 2 small features to be added which the team has informed us they are working on. One being different quantity upsells (buy 3 for the price of 2 etc) and the other being able to fire the popup on load of the cart page, not only when clicking the "checkout" button.

For anyone wondering why they should pick this one over their other popular upsell app Candy Rack, it is because with this upsell app different levels of discount offers are STACKABLE. It is also better to fire upsells at checkout rather than at add to cart click, because add to cart action is better for lead collection such as FB Messenger opt-in so abandon cart sequences can be sent later etc.