You can save a lot of money with carsdirect.com, but having recently gone through a carsdirect.com transaction with a friend of mine, I can tell you that experiences vary greatly. In short, when she went to pick up the car, the carsdirect.com rep hadn't delivered any of the proper paperwork to the dealer and was extremely hostile (when we could actually contact him.) After about 3 hours of trying to resolve all the issues, the dealer paid carsdirect.com to back out of the transaction so they could sell to her directly. Seems that the quality of the local carsdirect rep can vary considerably.

As for oil change intervals, call around to your local car parts shops and see if they sell oil test kits. You can send in a sample of your 7500 mile oil, and see if it needs to be changed more regularly. You may be surprised- I found that even with hard city driving, I could get at least 10k miles between changes of Mobil 1.