Do your research to avoid car rental shocks

In future, sites like Flightcar.com will rent your car to travellers if you park it at their airport booth while you fly away.

Compare and then double-check prices online before hiring a vehicle.

Rental cars are costly. If the upfront hire fees aren't enough, there are lots of other charges that can double the cost. That can be anything from increased insurance costs to reduce excesses, to hiring a child seat.

If you know how the system works, however, it's possible to minimise costs.

If you're booking a hire car, the first thing to do is to use comparison engines that list more than one rental car company. Beware, however, that you may not be comparing apples with apples. Do they offer the same number of included kilometres, what does an excess waiver cost, is roadside assistance included and are there extra fees for dropping off and picking up at an airport or one-way hires? What is the cost per litre of petrol if you don't leave the tank full, are you charged extra for a second driver, and so on?

Always double-check prices against what's on offer on rental car company websites. They may be cheaper.

Watch out for added extras at booking time or when you pick up the keys. In some countries staff at rental car companies get commission on the extras and employ high-pressure selling tactics for upgrades and other items.

For example, an infant capsule costs A$91.26 (about $102) to hire for seven days from Europcar on Australia's Gold Coast. It may well be cheaper (and safer) to bring your own child car seat - even if you have to pay excess baggage on the flight.

Insurance is a big issue. Hire fees cover basic collision and theft insurance, but excesses can be up to $5000, which is a lot to pay. Collision damage waiver cover from the rental company can almost double the cost of a hire.

If you're travelling overseas then your travel insurance probably covers the rental car excess. But beware that rental vehicle excess insurance does not take the place of rental vehicle insurance. It simply covers the excess.

"Travel insurance rates are cheaper and will simultaneously cover you for medical expenses, lost luggage, cancellation and more," says Natalie Ball, director of comparison website Travelinsurance.co.nz. Travel insurance has limits, says Ball, which are typically up to $5000.

The cover on credit card travel insurance may not be as high as a standalone travel policy. BNZ's Gold Card insurance covers $2000 a journey. Westpac's covers $3000 a family. If your excess is higher than that you could be left out of pocket.

Thanks to ACC and our home contents policies that usually cover us anywhere in the country, Kiwis rarely think about taking out domestic travel insurance policies. Domestic policies from insurers such as Cover-More often cover rental car insurance excesses.

If ever there was a reason to read the fine print of both insurance and rental car hire policies, it's with rental car excesses. Some travel insurance policies won't pay out the rental vehicle excess cover if you break the rental agreement. Some insurers decline claims if you break the road rules. If you're speeding and cause an accident you might kiss your cover goodbye.

There are some policies that require you to have "fully comprehensive" cover with the rental car company before your insurance excess cover pays out.

The fine print of the hire agreement is also important. For example, it's not unusual for a rental car company excess not to cover the under-body and/or roof if they're damaged in a collision with a bridge, tunnels or overhanging structure. It's unlikely your travel insurance covers this.

And if a third party claims damages against you for things such as loss of profit, you're on your own unless it's covered by a travel insurance policy or New Zealand contents policy. Not all do.

One Kiwi traveller, Olivia, found there was a costly gaping hole between her rental agreement and her travel insurance policy. Her New Zealand insurer picked up the excess for a car accident in Australia, but refused to pay a "loss of use" charge of A$696. Olivia appealed to Financial Services Complaints, which agreed with the insurer that consequential loss was not covered by its policy. Olivia would have been covered if she'd paid for the car hire company's additional cover as well as travel insurance.

Also beware that in the US, vehicle insurance often covers car owners for rental cars. So the cheap rate you're paying may not include any insurance at all. Ouch. Make sure, as well, that you're covered for legal liability from any accident you have.

Using your credit card to book a rental car can prove problematic, but most companies demand it. The rental car company will often hold the excess for your car, which can eat up all available credit, meaning there is no money to actually do anything on holiday.

Take a look at social media and you'll soon come across people who returned home to find all sorts of unexpected credit-card charges.

It's safest to use a debit card without a credit facility or pay cash. But not all car hire companies accept this. Europcar said a straight "no" when I asked this week. Apex Car Rentals said it would take a bond of the equivalent of the excess, but would refund this on a car's return. At least there would be no unexpected charges on the credit card at a later date that way.

Southern Cross' chief executive Craig Morrison has a good tip if you want to avoid being scammed by car hire companies over damage. Do your own pre-inspection report by videoing and photographing your rental car when you pick it up, and repeat it when you return it. Make sure the date stamp is on and, if possible, get a staff member in the picture. Morrison says he has seen cases where rental car companies try to make a quick buck by getting customers to pay for damage they didn't do.

When it comes to saving money on car hire, relocations are a great source, says Emma Gardiner, general manager at Thrifty Car Rentals. A one-way hire from Queenstown to Christchurch, for example, might cost you just the petrol and insurance.

This can be available overseas, as well, either directly from car hire companies or through relocation websites such as Transfercar.com.au. Earlier this week it had $1-for-two-days deals to transfer a campervan from Sydney to Melbourne, and an opportunity to pick up a campervan in Alice Springs for delivery to Cairns within 18 days for free. This company also operates in New Zealand. In the United States try imoova.com or just search "rental car relocation" for the country of your choice.

The rental car industry is heading for disruption in the way that hotels are under pressure from Kiwis renting out rooms in their homes on Airbnb.

The YourDrive.co.nz model enables Kiwis to rent cars by the hour or day from people in their vicinity. Travellers can do the same in other countries through websites such as RelayRide in the US, DriveMyCar in Australia and Mitfahren in Germany.

It's not in New Zealand yet, but Flightcar.com has a brilliant scheme. Instead of parking your car at the airport and paying a fortune in parking, you rent your car out to travellers while you're away and get paid. FlightCar organises the insurance for the hire period.

When someone starts a FlightCar-type operation at Auckland Airport, I'm signing up. My car would be in the rent-a-dent category, but $50 a day in my pocket would be better than paying to park.