It would be an online car parts store. When you order something, you give a scan of your license and registration (to be sure that YOU own the car), a photo of the car with the license plate visible, and a detailed description of where the car is parked and where you will leave the keys. Then, late at night, a team of mechanics will come to your house and install your new parts for you to wake up to the next morning.
For bigger jobs that will take more than a few hours, you'd give a certain time frame in which you don't need the car. It will be taken away one night (and a note will be left at the door and via email, so you know it wasn't stolen) and then it will be dropped off discreetly when it's finished.
Okay, I know that most of us prefer to do all our own work, but wouldn't it be cool to just wake up one morning and see that your car is suddenly better than when you left it? Plus some jobs are just annoying. For example, I think I'll have to remove my rear wheels to install my new mudflaps, and that's just tedious.