[Greyfriars]

If you are just using the toll roads back and forward to the airport just pay cash. We use them all he time, so use the Alamo toll system. with this there is no sunpass but a photo is taken of your reg plate and they use this to bill you after you come home. the cost is $3.95/day plus toll. The maximum is $18.95 plus tolls for the holiday period.
Some schemes, notably Dollar and Sixt are very expensive so i would only pay cash with them.
If you have no change on arrival at the airport ask for some quarters at the first toll booth as you will need exact change on most of the exit slip roads. The toll booths accept up to $20 notes.