A cross-sell occurs when an existing customer purchases one or more other (ie an additional) products or services. Cross-selling is important because it increases the total value of a customer (CLV) but without the additional costs associated with brand new customer acquisition (CAC).. Cross-sell and upsell are often used together, with product features that are optional but add significant value to the core product offering. Cross-sells include the sale of additional services and products to another area of the client’s business.