You know the routine. After a frenzied flight, you scurry to the car rental counter eager to claim your ride and get on the road to summer vacation. "Initial here, sign there," the clerk tells you, as other customers wait impatiently behind you.

But before you take up the pen, here's a heads-up: Tack-on fees, together with taxes, can boost your car's quoted rate by as much as 50 percent.

See also: When to get the best deal on a car rental.

Avoid rental car rip-offs by paying attention to fees and taxes. — Derek Shapton/Gallery Stock

Such fees are revealed deep in the fine print of the contract, which many renters don't read during the check-in process. And those who try to spot them in advance on the company's website discover that they're not exactly easy to find.

"It used to be that car rental companies made it as easy as possible to get you signed up and on the road. These days, consumers need to slow things down and be more guarded to make sure they don't pay more than necessary," notes Jeff Blyskal, senior editor of Consumer Reports, which along with sister publication ShopSmart recently examined rental agency practices.

To keep your bill down, keep your eyes peeled for these forms of rental highway robbery:

• Unexpected add-ons. Beware of features that you may assume are free. Satellite radio can cost an extra $5 a day. A GPS navigation system runs about $50 a week. A company-provided electronic toll payment device costs extra — sometimes with a second surcharge for "processing" your toll charges. A recently filed class action lawsuit alleges that one rental company charged a "service fee" amounting to 16 times more than the tolls run up by renters.

Your defense: When making a reservation — and again at the pickup counter — ask about every possible cost. In addition to equipment, this includes late charges (fees typically apply 30 minutes after your car is due back), mileage limits and roadside assistance, which was once free but now costs extra.

Using your own toll device is usually fine, provided it works on the highways you'll be driving. But to be totally safe, call your provider's office once you get your rental and give the car's license plate number.

Next: Is the car rental add-on insurance policy necessary? >>