You can find it at STEP3 in the online booking process under EXTRAS. Here you can reserve extras such as baby seats, GPS or add Additional Drivers. The price of the extras are displayed but these are paid for on arrival, at the desk. Certain car models do not display all of the extra options, so experiment by choosing a different car model if the extra you require is not offered. Finally, at STEP2, where the list of cars photos are displayed, look out for any offer icons, as often GPS or Additional Drivers are offered free.