For Shopify Plus members, you can sell products in your checkout. By using this feature, you can select the products you want to sell.

As a user adds a product to their cart, and moves to the checkout, this will recognise the products in the cart, and serve up the recommended products.

These recommended products can be populated via a product page in the CMS and done on a per product basis - this allows you to upsell specific products that pair will with whatever is in a users cart.