Linked Products add-on for WordPress Download Manager will add cross-sell and up-sell feature with Premium Package add-on.
Up-sells are products that you recommend instead of the currently viewed product. They are typically products that are more profitable or better quality or more expensive. Depending on your theme template, these products display on the single product page underneath the product’s description.
Cross-sells are products that you promote in the cart, based on the current product.
They are typically complementary items. For example, if you are selling a laptop, cross-sells might be a protective case or stickers or a special adapter. Or if you’re selling a ninja t-shirt, they could be a ninja hoodie and ninja socks.