What price comparison sites don't tell you

I am an idiot. But, assuming you have not yet gone on holiday in Europe this year, I would like to help you be less of an idiot.

I ended up booking our car hire in Italy on the internet just a week before we left. By that time any of the vaguely familiar names in the car rental business were asking £400-plus for a car for the nine days we needed, before excess insurance and the like.

Goldcar Rentals was asking significantly less (£171). That seemed like a deal. The final price was £207.50, because I somehow failed to reject the offer of damage excess insurance from Rentalcars.com (this involved unticking a box).

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I was irritated by the latter I hate being duped into buying extras on websites but not really angry. I know it is insanely overpriced, but I often buy excess insurance from car hire companies.

I know I am paying far too high a premium for the tiny risk that I might damage a car, and in my rational mode, I wouldn't advise you to buy it, but the truth is that on holiday, I'll pay an irrational amount of money for peace of mind.

When we got to Rome, the magnitude of my mistake began to dawn on me. We waited for 45 minutes; put up with a trying hard-sell for more insurance; paid a stupid amount of money for the compulsory purchase of a full tank of petrol; left our credit card details; and eventually set off.

The car itself was fine, assuming you don't mind going up all hills in first gear. But when we got back to return the car, we had to wait in another long queue. Just as I was about to get a tiny bit cross, the woman behind the desk got to us and went out to check our car.

I watched her carefully checking the wrong car. I listened to her telling me all was well. I told her she had checked the wrong car. No thanks received, but off she went, only to return and say the correct car had a flat tyre. Indeed, it did.

Concerned that the bill for this not very flat tyre would shock me into the arms of Avis forever, I asked what the cost would be. She refused to tell me. I persisted. Eventually she gave me a photocopy of a charges list with the relevant line highlighted. Tyre: €180. "You cannot be serious!" Shrug.

The flight was soon to close. The children were flagging. So, we gave up, as I (and probably the Goldcar business plan) assume most people do. When I got home, there was already an email waiting for me. Attached, it said, is "a message of interest to you". The flat tyre had been upgraded to simply "wheel rim". The new cost? €360. No explanation. No photos.

I went back to the website from which I had inadvertently bought insurance to cover myself in such an eventuality. Guess what? "Windows and glass, tyres, undercarriage, replacement locks, replacement keys and towing charges" are excluded from the insurance.

I'm waiting to hear whether "rims" comes under "tyres". But by Thursday this week, it looked like renting from Goldcar and buying my insurance from Rentalcars.com was to cost me £547.49, including £87.50 for the less-than-half a tank of petrol I used, when the real cost for a full tank would be £55. I could have had an Alfa Romeo Guilietta from a better-known agency for less.

My story has a happy ending. After I raised hell on Twitter, I got an email on Friday morning saying that "wheel rim" was the "wrong concept". It has been re-re-classified as a "puncture", the price for which has happily come in at a mere €60.

I'm pleased. But I do have to wonder if the same speedy reclassification and refund service is available to unhappy clients who aren't columnists for national papers with lots of Twitter followers.

You might say that I should have known better, given that I've been writing about opaque pricing and its effects for 15 years. Yes, I should. I know perfectly well that leaving booking too late, going on price alone and not concentrating when you book online is the kind of thing that often ends in an expensive fiasco. But I did it anyway.

The lessons here are boring, but important. Goldcar clearly has issues with clarity and customer service. But as much of a problem here is the false sense of security we get from price comparison websites.

Look at a comparison site for anything and until you have done an awful lot more work, it doesn't really tell you that much. The UK's financial regulator is already looking at last-minute add-ons such as damage insurance and legal protection, which it likens to "Creme Eggs at the till".

This week, it pointed out that most users of price comparison sites buy on price alone and don't consider the extras or whether the product is appropriate.

That review was about insurance, but the same principles apply elsewhere. With car hire companies, you need to factor in the extent to which insurance and fuel may be overpriced.

With bank accounts now subject to yet another investigation you need to check how much your overdraft will cost you, something that Which? says is "almost impossible".

With savings accounts you need to figure out how long the bonus lasts and how much interest you lose if you withdraw your own money.

With investment funds you have to work out how much trading costs add to the advertised costs.

With investing platforms you need to do a series of complicated calculations to figure out the potential cost of your portfolio on each site, because they all charge different amounts for different things.

It isn't in the interest of car hire companies to tell you the real cost of driving, any more than it is in the interest of fund managers to tell you the real cost of investing. I wish that would change. But the other thing that the past 15 years has taught me is that it never will.

This article was first published in The Financial Times.