Somedude24 is right, all domestic US airfares are subject to a 7.5% excise tax that MUST be displayed as part of the base fare per DOT regulations. So $99 is really a $92.09 fare plus a 7.5% excise tax of $6.91. Then after the display, PFCs and security surcharges are added in. The reason these are added later is because the total PFC and security fees are dependent upon the routing, while the excise tax will be the same no matter where you route on the fare.