For domestic and overseas travel alike, some of your journeys are likely to include renting a vehicle at some point. Nowadays , the choice is plentiful. You can book through your travel agent, directly from the car rental company(online or not) or through third-party sellers on the internet. This last method is becoming popular because it seems to offer some savings over the other methods. But one has to be careful for the kind of companies you are dealing with.
I am now able to advise you against one of these companies : Arguscarhire.com ( also doing business as car trawler.com). First a foreword: when I rent a car, I use all these THREE methods for car rentals depending on several factors ( Is there a rental company that I trust in the car pick-up area, have I dealt with this location before, do I need a specific vehicle,..)
Over the last 2 years, I had rented vehicles from this company several times before I found the issue. On first sight, the choice seems to make sense : the site is easy to navigate, allows you to pick the rental car company  (a MUST, in my opinion, when renting a vehicle), the pricing is competitive and clear BUT there is a catch, and a big one in my opinion !
Very often, when you rent car online, you have TWO different prices for one given rental  depending if you want a cancellable and refundable contract or not. It is the same as when booking hotels room online on many sites. As for me I generally prefer to pay a slightly higher price for the added flexibility, especially if the car rental is taking place during the middle of the trip. No problem since arguscarhire has a option with a 24 hour prior notice policy, you think : well yes EXCEPT Argus carhire does not honor its policy and will try every dirty trick in the book to prevent you from cancelling your reservation.
In July I had made three reservations with Argus carhire one in the USA and two in France. As it happened I needed to cancel two of them and was dumbfounded about the response from the company. For the US reservation, I tried to cancel online 36hours before but when logged in  got a message saying that this could not be canceled online as it was supposed to be. The “contact” button was strangely unresponsive. The company is based in the UK so calling from somewhere with a 8 hour time difference is both an expensive and unpractical option. I tried and got nowhere.I finally was able to get an email address and the got an answer THREE days later saying that the car cannot be cancelled AFTER the pick-up deadline. In other words, they just sit on the email until AFTER the deadline to inform you that this too late to cancel. What they tried to do for the French reservation was even worse and the emails I got were almost insulting with their content.
BOTTOM LINE is if you are a smart traveler and do not want to get screwed by a company which is dedicated of giving you phony excuses when you try to change or cancel the reservation when you are within the time frame to do so, do NOT use this company. I know I won’t . Not worth the hassle.