Quite common for many people to have problems with these rental companies. Heck, I am one of those people. I've tried many different rental car companies. Most of the time, the most problems I have come from the crappy staff they hire. The customer service is just horrid. Car rental agencies are a regular fixture at airports all around the world, each promising traveller's deals on automobiles to take them far and wide. However, many of those deals evaporate quickly when car agencies add on a number of hidden and blatant charges to a traveller’s invoice. Fees and deposits for damages, cleaning, road tolls, and more can blow a budget up without notice. So I ended up sticking with only one service that is way better than I expected after a bad history with rental companies, I'm talking about https://rental24h.com/usa/denver-airport/avis AVIS cars are edged with an orange frame. Also, there are cars from other companies and from neighbouring locations that will help you to choose the most appropriate at a price and other parameters option. To rent one of their cars was more than fast. Usually, it takes so long with all the documents and what not. Really handy for when I am in a rush and have no time to dilly-dally. Maybe you will find better luck here? I know I did.