To illustrate piece on safe teen driving event. Keywords: Convertible,...

To illustrate piece on safe teen driving event. Keywords: Convertible, Driving, Teenager, Car, Student, Adolescence, The Future, Little Girls, Teenagers ( iStock) For Explore LI / Valerie Kellogg - Beth Whitehouse�s liKIDS page LSKIDSRAIL20. Credit: ISTOCK/iStock

If you thought airlines had wacky rules, try renting a car.

Like airlines, car-rental companies are astonishingly creative with their fees, from additional-driver add-on costs to premiums for customers younger than 25.

Here are some odd rules commonly instituted by major car-rental companies that travelers should be aware of.

1. PRICING AT WILL

Some rental companies reserve the right to recalculate your entire rate if you return the car later - or even earlier - than what's spelled out in your original contract. The difference can be costly. Avoid fluctuating rates by returning your car on time or paying upfront through a site such as Priceline or Hotwire. 

2. AGE MATTERS

If you're younger than 25, a car-rental company may add a surcharge of $20 a day or more to your bill. If you're younger than 21, it might be impossible to rent a car. Auto-rental companies insist the fees are necessary because younger drivers are likelier to damage a car. Enterprise and Thrifty have more lenient policies for young drivers.

3. THE EXTRA DRIVER IS . . . EXTRA

If there's a second driver, car-rental companies often ask for more money - about $10 to $12 a day. Try to book through a discount website that allows you to prepay.

4. THAT REQUIRED CHILD SAFETY SEAT WILL COST YOU

Imagine if a car-rental company forced you to pay a "seat belt fee." That's a decision parents face every day when they rent a car and are unable to bring their own safety seat. "For the cost to rent a car seat, I can buy one," says Laura McDonough, a software project manager in Austin. It's a hassle, but you can still check a safety seat on an airline at no charge and then install it in your rental.

5. YOU SAY COMPACT, I SAY MIDSIZE

Pay close attention to car-rental classes. What we consider to be compact might be classified as midsize (and more expensive). Alamo reclassified its vehicles after being acquired by Enterprise.

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