The Visitor Oyster Card is an electronic card that allows travellers to store credit for pay-as-you-go travel on Transport for London services. The card automatically calculates the cheapest fare possible and stops deducting credit when a daily amount is reached. Your £20 card can be topped up with additional credit at Tube stations, Oyster Ticket Stops and London Travel Information Centres. Your Oyster card has been pre-actived so you can start using it as soon as you arrive in London.