You generally pay for your service once the service is complete if it's a same day repair. If it's a long term repair in the case of a crash and your car finds its way to a body shop to be repaired then you may need to make installment payments along the way as parts are needed. The dealership / mechanic may also refuse to release your car back to you if you don't pay the full bill before taking the car home. I have heard of that happening.