Planning to rent a car in Europe this summer? Fasten your seat belt: The worst sticker shock I faced during a recent trip to Spain was the rental car bill. Learn from my mistakes and save yourself some grief.

Guess how much I paid for my eight-day rental of a midsize, automatic-transmission car from Avis in Madrid (at the Atocha Railway Station branch)? A whopping $1,311. And that didn't include any insurance. When the Avis rental agent informed me that purchasing the collision-damage coverage would shoot the bill up to $2,211, I paid with my American Express card, opting to rely instead on the car-rental insurance it provides.

Since I am all too aware of the multifarious fine-print loopholes in American Express's coverage and the many hundreds of euros that renters can get fined for the tiniest scrape or dent, I was a nervous wreck for the next eight days. It didn't help that we—my husband, our two children, and I—ended up getting caught in downpours on unexpectedly rough roads, from the hairpin switchbacks of the Alpujarra Mountains to the cobblestoned alleys and tiny (even for Europe) parking spaces of Granada (see "Wendy Perrin's Worldwide Guide to Affordable Villa Vacations"). Thirteen hundred miles later, by some miracle, we returned the vehicle in downtown Madrid unscathed.

Not so fortunate was a Condé Nast Traveler colleague who also rented a car in Europe this past winter. He paid $495 for a four-day rental of an automatic-transmission subcompact from Hertz at London's Heathrow Airport and relied on the insurance provided by American Express. When he inadvertently put four gallons of gasoline into the diesel fuel tank, realized his mistake before starting the car, and phoned Hertz for roadside assistance—which came and purged the vehicle of the fuel—he assumed that AmEx would cover the cost. Not so. The roadside-assistance bill was $891.

Travelers to Europe, take heed:

Ask in advance about every conceivable additional fee.
Even when you're locking in a U.S.-dollar rate or prepaying for a voucher you hand over at the rental counter—as I did when I rented from AutoEurope in Italy—inquire not only about insurance costs but also about other potential fees. Because Avis charges no additional-driver fee for spouses in the United States, I had assumed that the policy would be the same in Europe. Wrong. The fee for my husband and me to share the driving was $79.

If you plan to pay with a credit card that provides insurance, read the coverage plan.
Before picking up the car, scrutinize your credit card issuer's insurance program—which should be available online—so that you know what kinds of damage, which types of vehicles, and which countries are not covered. AmEx, for instance, does not insure cars rented in Australia, Ireland, Israel, Italy, Jamaica, or New Zealand; rentals of more than 30 days; or luxury cars, full-size vans, or SUVs. And it covers you only when you are the primary renter—meaning it must be your name (not the name of your travel companion) on the rental agreement.