To start using post-checkout upsells, make sure the add-on Post-Checkout Upsell is installed and enabled at your store.
If this add-on is not installed for some reason, you can find it in the X-Cart App Store. General add-on installation instructions are available in the section Installing Add-ons from the X-Cart App Store of our Knowledge Base.
The add-on Post-Checkout Upsell has a dependency on the add-on Related Products because it uses the "related products" data type which is implemented by that add-on. For this reason, you may notice that the add-on Related Products is active at your store even though you have not chosen to enable it. You will also not be able to disable it as long as Post-Checkout Upsell is active.
After making sure the add-on Post-Checkout Upsell is enabled, you need to configure it. See Configuring the Post-Checkout Upsell Add-on.
You can also change the title of the post-checkout product recommendation popup block as you require. See Making Changes to the Post-Checkout Product Recommendation Block.
For product recommendations to start showing up in your post-checkout product recommendation popup block, configure the connections between the products in your store. The products that you want to appear as recommendations for a specific product will need to be specified as related to that product. See Adding Product Recommendations for Post-Checkout Upsells.