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.