Upselling and cross-selling can lead to a major boost in revenue by selling more to the same customer. Effective upsells/cross-sells are designed with the customer experience in mind.
Checkout is the ideal time to introduce an upsell or cross-sell, as your customers are already primed to buy. Customers who have already pulled out their credit cards are more likely to respond to an offer for a product that directly relates to their initial purchase.