Arguably one of the most effective upsell placements and recommendation sources you can use with Checkout Components, who gives a crap uses a simple upsell component that connects to a collection of products they are looking to sell that has been curated for quick conversion.
One of the biggest drivers of conversion in checkout is their user of the product information modal, which opens when a customers clicks on a recommendation image. This modal contains an image gallery, the Shopify product description as well as pricing information.