The practice of attempting to sell a higher-value product to the customer.

a product you offer them after they've bought from you

The sale of a product or service in addition to the product or service the customer has purchased. For example an agent may sell an additional feature such as voice mail with a new telephone service.

To try to convince a customer to purchase a more expensive product or service. For example, when you order a "combo" meal at a fast-food restaurant you may be asked if you'd like to purchase a larger size of fries or drink at a discount.

to sell customers a higher-priced version of a product they have bought previously

To encourage a customer to purchase a more expensive item, an add-on, or related product.