Generally, bookable products require some information about the user and or the individuals attending. Once the user has selected all the passes they are interested in, you must fetch a BookingForm and provide an interface for the user to complete this form.
Note that the Passes in the arguments is a flattened array of Passes. For example if the user requires two of a single pass (i.e. two adult passes) then the array holds two of the same Pass type.