Permit holders who want to buy multiple daily charges in advance must add one item at a time to your basket. For example, suppose you would like to pay three separate daily parking charges. In that case, you must click the ‘Add to basket’ button three times and wait between each click for the system to reset. Do NOT click the '+' button to increase the number of items to three before adding to the basket as doing this will result in paying for three charges for the same day.
Further details are available at https://travel.web.ox.ac.uk/car/permits#tab-3148666