Cross selling involves selling an additional product and services on top of the one that a customer has already agreed to buy (or has already bought).
In other words, cross selling is the process of selling a different product to a customer to increase the value of sales.
For example a staff of FreshOne fast food asking the customer if he wants fries with his burger. The fries are complimentary products to the burger.
Another example is where a Solicitor is employed to register a trademark on behalf of a client; the Solicitor may put himself across as capable of the enforcement of any infringement of the trade mark. For the same reason, where a Solicitor is engaged to purchase a property, the Solicitor may cross sell himself by offering to perfect the documentation arising from the sale.
One way to cross sell is to offer complimentary services to the customers.