Travel agency's main function is to act as an agent, selling travel products and services on behalf of a supplier. Consequently, unlike other retail businesses, they do not keep a stock in hand. A package holiday or a ticket is not purchased from a supplier unless a customer requests that purchase. The holiday or ticket is supplied to the agency at a discount. The profit is therefore the difference between the advertised price which the customer pays and the discounted price at which it is supplied to the agent. This is known as the commission. In many countries, all individuals or companies that sell tickets are required to be licensed as a travel agent.The majority of travel agents have felt the need to protect themselves and their clients against the possibilities of commercial failure, either their own or a supplier's. They will advertise the fact that they are surety bonded, meaning in the case of a failure, the customers are guaranteed either an equivalent holiday to that which they have lost or if they prefer, a refund.