It’s a nice feeling to find a great deal on a car rental, but before you jump for joy, take a step back and be sure you really are getting a deal. Car rental companies tend to make up for fabulous prices by charging hefty fees where you least expect them. Here are some tips for avoiding excess charges.

First, make sure you use the car for exactly the contracted length of time. Pick it up and return it when you say you will. If you don’t, you may be charged penalty fees. Worse, any change to your contract could automatically bump you into a new, higher rate schedule.

Next, keep an eye out for additional gas charges. Fill the car up prior to bringing it back to the rental company and keep your receipt as proof that you did it. Some companies may try to charge you for gas anyway, assuming you won’t notice. Take a moment to really read over your final bill and to dispute any charges that seem unwarranted.

Finally, be prepared if the sales representative asks you to purchase supplemental insurance. Most of the time, you’ll find your automobile insurance covers you in rental cars. Some credit card companies will also cover damage to rental cars. Sales reps can be very persuasive, but if you know you have other coverage, decline the supplemental insurance.

If you do decline the supplemental insurance, walk around the car with the sales rep. Make sure he or she makes a note of every scratch and flaw so that you won’t be charged unfairly when you bring the car back.

Little tricks and fees of car rental companies can change your “steal” to a “bad deal.” Always know exactly what you’re being charged and why.