Electronic toll payment is a hugely convenient time-saver, especially if traffic backs up in the cash-payment lanes. Unfortunately, if you’re vacationing in a rental car, the add-on service fees for e-toll tags can be hefty.

Hertz customers who use that company’s PlatePass e-toll transponders pay $2.50 per day in addition to the price of any tolls, up to a maximum of $10 per rental month. Avis and Budget charge $2.50 per day or $10 per week in fees, as well as the tolls.

Dollar/Thrifty e-toll rates vary by location, however they can add $6.99 per day or $36.99 per week in Miami. Although that price includes the cost of tolls, in case you drive down the “Alligator Alley” Everglades Parkway toll road from Miami to Naples just once, a $2.50 toll, you’ll still pay Dollar/Thrifty $36.99 on a weeklong rental-a 1,380 percent markup on the actual toll. (Jim Glasheen, a Rent A Toll spokesman, counters that you can also conceivably rack up $38 in Florida tolls in one day yet still pay only $6.99.)

E-toll tag fees are among many rental-car gotchas utilized by national and local chains you need to know about. For example, one Consumer Reports reader felt duped when his credit card was charged $9.50 for $2 in New Hampshire tolls through PlatePass earlier this year. A Consumer Reports editor was similarly surprised to find $10.78 in Budget toll charges and fees on his credit card statement a month after he settled payment on a Texas car rental last year.

What you can do

Car renters who wish to zip through the electronic toll lanes can take several steps to prevent being gouged: Use your existing e-toll account. If you currently have an EZ Pass, FasTrak, or SunPass account with a portable transponder, bring it with you whenever you rent a vehicle elsewhere inside those e-toll systems. For example, an EZ Pass account holder in New York can use his transponder in 13 other northeast states, from Maine to Illinois to Virginia-but the device won’t work on California’s FasTrak or Florida’s SunPass systems, says Ken Philmus, a senior vice president with Affiliated Computer Service, the organization that operates the EZ Pass system.

Get a temporary pass.

Some states allow you to buy a temporary e-toll account, so search the web for the transportation department of the state or states you’re visiting and check their bridge and highway e-toll options. The North Texas Toll Way Authority, for example, will let you set up a temporary Zip-Pass account with a minimum $20 credit card deposit, and unused deposits are refunded as soon as your account lapses. The Florida Department of Transportation also sells temporary SunPass mini stickers that work the same way. But Nicole Kalil, a department spokeswoman, cautions travelers to ask their rental car agency when they are allowed to affix the light-adhesive sticker to the windshield, and consumers should be certain to take off the tag whenever they return the automobile.

Demand a refund.

Check your credit-card statements in the month or two following a car rental. If you find e-toll charges that were never properly disclosed to you, agree to pay the tolls but demand a refund of the hidden service fees. Consumers shouldn’t be charged without proper prior notice of the threat of fees, their amount, and when and exactly how fees are triggered. In the event the rental agency or e-tolling company won’t budge, dispute the charge with your credit-card issuer as an unauthorized charge.

Wait ’til next year.

In case you rent a vehicle in states not covered by the tips above, be patient. Electronic tolling authorities from across the country are creating a system that will allow one e-toll system to bill another system’s customers directly, without penalty. The government-run system will be tested this fall which is anticipated to be operational at the beginning of 2012.

“If there are any consumer fees associated with our system, they will be small. We’re looking at cents per transaction,” says J.J. Eden, president and CEO of the Alliance for Toll Interoperability. That exposes how much rental car consumers are overpaying today for e-tolling services.