The WooCommerce Boost Sales is the best upsell and cross-sell plugin. The main objective of this plugin is to increase sales and revenue by improving the customer shopping experience. Along with upsells and cross-sells, this plugin also enables a discount bar on your website, through which you can show customers the spent amount required to avail of the discount.
With this WooCommerce upsell plugin, you can display popups when the customer enters products into the cart. You can show the upsells in 3 different settings: based on a similar category, recently viewed products, and frequently bought together.