What are the best cross and upselling apps for Shopify?

There are many apps available for Shopify integration and for making sales from it.

You can use Shirtee.Cloud to add print on demand products directly to your Shopify store within seconds for selling.

Also, you can use Bold Upsell for pop-ups and funnel that displays up to three rounds of different items and increase upsell.

Another useful app is Frequently Bought Together by which you can display recommended products.