Average order value is one of the most important numbers in e-commerce. Increasing it by 15 percent through upselling costs nothing in customer acquisition. WooCommerce has basic functionality built in, and plugins make it better.
Built-in features
The product page has fields for "Upsell products" and "Cross-sell products". Upsells appear on the product page: "You might also like..." with more expensive or better alternatives. Cross-sells appear in the cart: complementary products.
Automatic recommendations
WooCommerce Product Recommendations (official, 79 USD per year) analyzes purchase history and shows recommendations based on what other customers bought. "Customers who bought X also bought Y." It replaces manual configuration per product.
Order bumps
A checkbox at checkout: "Add extended protection for 99 SEK" or "Add gift wrapping for 49 SEK". CartFlows, FunnelKit, or WPC Frequently Bought Together. A clickable checkbox converts better than an extra product page.
Bundled products
Product Bundles (official, 79 USD per year). "Complete package: shirt + hat + socks, save 15%." The customer gets a better deal, you sell three products instead of one. Present it as an offer, not as a manipulation technique.
Don't be annoying
A popup blocking checkout with "Did you forget this?" isn't upselling, it's an obstacle. Show recommendations in the flow (product page, cart, checkout) without interrupting the purchase process. The customer should be able to ignore them without frustration.