Three different mechanisms, often confused.
RELATED PRODUCTS
Generated automatically from shared categories and tags. You cannot choose them directly, but you influence them by how you categorise and tag.
UPSELLS
Products you suggest instead of the one being viewed, usually better or larger versions. Set on the product's Linked Products tab. They appear on the product page.
CROSS-SELLS
Products suggested alongside what is already in the cart, usually accessories or complements. Also set on Linked Products. They appear in the cart.
USING THEM WELL
Cross-sells earn the most for the least effort. A customer buying a phone case is receptive to a screen protector at the cart. Keep suggestions genuinely relevant. Irrelevant suggestions are ignored and make the shop look automated. Three or four suggestions, not twelve.
SETTING THEM UP
Start with your best-selling products and add cross-sells to those. That covers most of your orders without editing the whole catalogue.
Review periodically. Suggestions pointing at discontinued products look careless.
