I just experienced renting a car at the Enterprise at Ontario airport - worst customer service. We had a reservation and booked a car online with a specific time. We arrived to check our reservation and Enterprise did not have a car the size we reserved online available. They proceeded to tell us we needed to pay an extra ~$15/day for the next 10 days for an upgraded car. I felt if Enterprise did not have the car size I specified in a reservation available, then they would offer the next available car at the same price. I expected an offer of an automatic upgrade and no additional charge. Instead, we were told to wait until another car came into the lot (which could have been minutes - hours). I asked to speak to the manager and was declined saying management had nothing more to say. I was definitely upset with this type of customer service and I am convinced they purposely do not keep cars in the lot in order to make more money off people in a hurry/needing a car. I had no problem walking up to the neighboring Hertz counter and requesting an available car. At that point Enterprise associate ran up to me asking why I was switching companies and after telling him what I expected from a company, he finally proceeded to give me an upgrade at no additional cost. This process took about an hour. I will not be utilizing Enterprise Services in the future. If this happens to you, my suggestion is to wait out until they give into a fair upgrade.