The rental car facility is outside the toll booths on the south side of the airport. No tolls are required.
Back in the old days the rental cars were inside the toll booths and they gave you a voucher to hand in at the booth.
If you enter/exit the airport at the south end (TX 183) you will be fine. If you want to exit or enter at north end you can stay on the service road and avoid the toll (but watch your speed).