Free job
When you are in job mode, press [Insert] + [Home] + [Page Up]. The job will automatically finish, you get to keep your money, will get paid.
Fix parts
To fix any and all repairable parts while they are still on the car without having to take them off, press [Delete] + [End] + [Page Down].
Free parts
Take parts off people's cars then enable the "Free job" code. You will still have the parts under that car name and still get paid the full amount.