They will not hit you with any extra fees when you pay for the rental. Except for any damage done to the vehicle. Rental car companies rarely charge for minor scratches due to bugs, door hits, etc... any longer but they used to do a walkthrough with you at the vehicle to note and pre-existing damage. Now, you go to the lane, pick your car and off you go. If you notice any dings or something major then please note it with them before signing anything or leaving with the car. Other than that and an empty tank they should not hit you with any extra charges.
We rent a vehicle for each and every single trip because we hate Disney Transportation. And we always get hit with a toll booth since we go off property about once a day and
sometimes that involves a toll road. We got a Sunpass system for $25 and just load it each trip with around $10 to $15. It clips onto the car and allows you to just fly through the Sunpass lanes. Easy peasy lemon squeezy. Trying to pay cash is a nightmare and it would surprise me to find that Sunpass has done something to the machines there to make it that way so you are forced to get a transponder.

The machines either take exact change only or the bill reciever is damaged or it just doesn't like that particular bill you are trying to use, etc...
#1, If the rental agreement says $168, then that is all you will pay as long as it has no damages and you bring it back filled up.
#2, Go for a transponder if you plan on going off site (to the beach or somewhere down a toll road) often but do not get the temporary one. It sticks to the inside of the window and the rental car company can charge you for removal.
***EDIT** Also, when you get the transponder (order it in plenty of time to get to you in the mail) don't worry about using it on a rental car. Before you leave the garage, or at least before you get to a toll booth, call Sunpass and give them your transponder info and tell them that you are using it on a rental so it will be activated. They will ask for the VIN number and maybe the plate number (I can't remember) and then you can drive through the Sunpass lanes with no problems.