What's the best way to add upsells and cross-sells on Shopify?

Match the tactic to the moment. On the product page, 'complete the look' / frequently-bought-together blocks work well. In the cart, a small 'add this' nudge or a free-shipping progress bar raises AOV. The highest-converting and least-intrusive is the post-purchase upsell — an offer shown after checkout that adds to the same order in one tap, so it doesn't hurt your conversion rate.

Apps cover all of these (post-purchase upsell apps use Shopify's native post-purchase extension). Keep it relevant and light — irrelevant or aggressive upsells annoy buyers and can drag down speed. Start with one well-targeted upsell, measure AOV, then expand.