At one time or another, we have all walked into a car rental office and met with one of the counter staff on a mission to hard sell insurance. It's like a knee-jerk reaction. See customer, sell additional insurance. The most common add-on is called loss damage waiver (LDW) and that can seriously boost your daily rate. So what is this mystery product and should we think about buying it? Well, let's start off with a few of the basics. LDW is a kind of get-out-of-jail-free card, covering you if you put a dent in the body work or run the car off a cliff. No matter what happens, you're off the financial hook. Most people own a vehicle of their own, have an insurance policy, and work on the basis this will cover them when driving a rental. But the $64,000 question is what cover you carry over. Let's start with the deductibles. To get the lowest possible premium on the regular policy, most people opt for the highest deductible. They reckon they are careful drivers and can afford to self-insure the first $1,000 of any damage. Except this does not quite square with the pricing policies of rental companies. Most seem to have in-house body shops paying top rate for repair personnel or use the most expensive independents. Although you might buy the cheapest possible replacement parts, your bill from the rental company will come in at the top end of expectations and add on the much-feared "loss of use" charge. This is their estimate of the daily loss of profit caused by not having the car available for rent. And, guess what. The rental company does not feel under any pressure to get the car back on the road. Suddenly, your deductible has gone and you find your own policy does not cover the loss of use charge.

But you're still not panicking because you remember your credit card company offers some kind of back-up insurance. Now's the time to read all that small print, i.e. before you rent the car. The terms often fall into the so-called secondary insurance market. In theory, this covers you for those heads of claim not covered by your own car insurance. Except the world never seems to work out quite the way you expect. What works on the Gold and Platinum cards may not work on others.

Auto insurance is never an exact science but there are one or two simple rules. If you are only renting for one or two days, it's probably better to buy the LDW because any claim you make does not show up on your own policy and you avoid any premium hike. But there comes a point when the daily rate is too big a hit. Now you are gambling you will not have an accident that takes the rental car off the road for a long time. The reality is the daily rate for loss of use probably will not fall under your own auto insurance and may not fall under the credit card secondary cover. So just make sure you only have minor accidents.