recheck your math. you were right the first time. $6 per 8 hours after the first day. Three 8 hr time periods per day... $6 times 3 does indeed equal $18 per day (24 hour period of time) after the first day. Still $18 a day, $6 for the first 8, $6 for the second 8, $6 for the third. now if you got to the lot before 16 hours went by on a particular day, you'd pay only $12 for that day, or $6 if under eight hours.
dont be fooled