Bold Upsell offers the ability to create both upsell and cross-sell offers.
Upsells are offers that will replace an item in a customer's cart. For example, a customer has added a 1-pound bag of coffee to their cart, triggering an upsell offer. The offer is to purchase a 2-pound bag of coffee instead, for only $3 more. If they accept this offer, the 2-pound bag of coffee will replace the 1-pound bag.
Cross-sells are offers that get added alongside products in a customer's cart. These items are often complementary to an item that a customer has just added. For example, a customer has added a bag of coffee to their cart, triggering a cross-sell offer. The offer is to purchase a pack of coffee filters.
Cross-sells don't necessarily need to be discounted to be successful; sometimes, reminding a customer of an item they may not have thought to purchase is enough to convert somebody.
For more information, check out the How to use cross-sells and upsells to increase revenue on Shopify blog post.