Cross-sell block
The 'goes with this' row: complementary items suggested next to what the shopper is already buying.
See it
What it is
A cross-sell block suggests items that go with what the shopper is buying: the lens cap for the camera, the socks for the boots. Its cousin the upsell suggests a pricier version of the same thing. Amazon's 'frequently bought together' and fashion's 'complete the look' are the two canonical shapes, and they are built from different data: co-purchase history versus a stylist's manual pairing.
Placement decides everything. On the product page it competes with the add-to-cart button, so put it below the fold. In the cart drawer it catches a shopper who already decided, which is why it converts best there. Inside checkout it becomes an order bump, and after payment a post-purchase upsell.
Gotcha: most default 'related products' widgets just show more items from the same collection, which is not a complement, it is a competitor. You have offered the shopper a reason to reconsider the thing already in their hand. Curate real pairings, and let the block add to cart in place with a quick-add button; sending someone to a new product page loses the original intent.
Ask AI for it
Add a cross-sell block to my product page and cart drawer. Title it 'Pairs well with' and show 3 to 4 complementary products in a horizontally scrollable row of compact cards (image, title, price, quick-add button). Source the items from a manual 'pairs with' list on each product, falling back to co-purchase data, and exclude anything already in the cart, anything out of stock, and anything in the same category as the current product. Quick-add must add the default variant to the cart in place with an optimistic UI update and no navigation. Keep it visually secondary to the main add-to-cart button and place it below the fold on the product page.