Whether you are traveling for business or pleasure, a rental car is sometimes a must. This option gives you more flexibility to explore your vacation destination, makes it easier to get your luggage to the hotel, and can make for a more stress-free trip in general.

But drivers who rent a vehicle might get an unpleasant surprise when their bill comes in. On top of the daily rate and other expected expenses, they may have unknowingly racked up a number of other charges.

Researching car rental policies ahead of time will let you know what the company expects of its drivers. It will also help you avoid purchasing any unnecessary extras or driving in a way that inflates your bill.

Insurance is a key consideration for rental car drivers. The Insurance Information Institute says companies will offer coverage such as collision damage waiver, liability insurance, and personal effects coverage.

In many cases, a driver's auto insurance will extend to cover any vehicles they rent. If so, it is not usually worth the added expense to purchase additional coverage from the rental car company. However, it is a wise investment if you do not own a vehicle and therefore don't have any personal auto insurance.

You'll also want to make sure there aren't any gaps in your insurance. If you have dropped certain coverage, such as collision insurance, you may want to purchase the extra protection. Call your insurance company to make sure you are covered for potential damage to rentals.

Even if you don't have auto insurance, your credit card may offer some protection. Call the company to see if they will cover collision damage or offer any other insurance benefits.

This coverage will only go so far. Jessica L. Anderson, writing for Kiplinger, says credit cards won't offer liability insurance. Instead of risking the high costs if you are found to be liable in an accident, you may want to purchase this coverage as part of the rental agreement.

Even if you are adequately insured, you might still be hit with a "loss of use" charge if you damage the rental car. This fee is meant to cover the revenue the company loses while the vehicle is getting repaired. You could also be charged for the vehicle's loss of value after an accident.

For this reason, it is a good idea to give a rental vehicle a visual inspection before taking it off the lot. Mark Kahler, writing for About, says you should notify the company about any scratches, chipped glass, or other noticeable imperfections. If you don't, the company might assume this damage was your fault and charge you for it. You can also take pictures of the vehicle stamped with the date and time to give evidence of pre-existing flaws.

Rental car companies ask drivers to notify the police and fill out an accident form if they are involved in a crash. Eric Jordan, writing for Conde Nast Traveler, recommends reporting even minor damage, such as a scrape or dented bumper caused by a poor parking job, to the police. He says that under some rental agreements, the repair costs might fall on you rather than the insurance if you don't take this step.

Drivers are required to return a rental car by a certain date and time, and failing to do so can lead to extra costs. The Federal Trade Commission says many companies give a grace period, but this typically only lasts for half an hour or so. If you bring back the car after this time, the company will levy an extra charge; if you are especially late, they can even charge you for another full day.

Rental car companies will also ask drivers to fill up the gas tank, or at least bring the fuel to a certain level. You may have the option to pre-purchase gas, but you generally won't be refunded for any fuel you don't use.

Airports are a convenient place to pick up a rental car, but it can be much more expensive than if you got the keys elsewhere. Kahler says airport surcharges cover the more expensive cost of doing business at these locations, and that the taxes alone might be twice what you would pay at an off-site rental agency.

If you come down with a flat tire or other problem on the road, a rental car company might address the issue with a roadside assistance plan. However, they will sometimes charge you extra for this service. If you currently have a roadside assistance plan, check to see if it will be able to help you out first.

Drivers can purchase a number of add-ons for the vehicle, but these will usually come with a daily fee. Some items include car seats, ski racks, GPS systems, toll passes, and satellite radio.

You may also be required to pay more depending on how the vehicle is driven and who is behind the wheel. Companies typically have fees for anyone besides the renter who will drive the vehicle. The FTC says people between the ages of 21 and 24 can rent a car, but typically pay an "underage driver" fee to do so. Your bill may also go up if you exceed a mileage cap, take the car out of state, or return it to a place other than the site where you picked it up.

Before you leave for your trip, research your insurance coverage as well as the rental agreements from different companies. Doing so will let you compare rental options and see which one best fits your needs.