Stay away from rentalcars.com!
I booked a car from them in May 2013 for a Heathrow, London pickup in late July. I cancelled my booking July 1st as I need to change locations and dates. I re-booked again with new dates, picked up the car as planned and used it for 2 weeks in the UK. When I returned to Canada I noticed that the credit for my initial cancellation didn't show up on my visa statement. It took over two months and numerous enquiries and e-mails to finally get my money back. Next time I'll book directly with a rental company!