By add-ons, I mean that you graduate the client to larger or superior alternative product or package of goods and services. In other words, they may have been content coming into buy the standard, basic product or service. And yet, by under-standing what their intended use for that product or service is, you realize that the basic, standard purchase can‘t possibly give the client the performance outcome they seek. So you recommend a larger, higher-quality, or more sophisticated version of the product that you know will give them a better result. They don‘t have to buy it, but you have an obligation to demonstrate to them the differences in performance and outcome they can expect to receive and to make them an offer that gives them an incentive for considering trading up.
A cross-sell is introducing to the client an additional product or service that will add or increase the result of their transaction with you or your company.