UR: Frequently Bought Together

Setting related products one item at a time stops working past a few hundred products. Set them from the product list without leaving the page, copy one product's set and paste it onto others, or add collection rules that cover a whole collection at once. Products you haven't set show Shopify's automatic recommendations, so the section works from day one. Shoppers tick the items they want, see the total, and add them to the cart in one click. Works on product and cart pages, in 20 languages.