Repairs not properly done

I was in an accident in May and I sent my vehicle to Technostar for repairs. It took them months to return my vehicle and at the end of it all my sensors( collision) were not working. when I enquired about that they rep**** that Mercedes would fix the sensors during the next service. I sent my vehicle this morning for a service and Mercedes is charging me for the sensor replacement as well as fees for stripping the vehicle bumper, which by the way was the very bumper that you guys were repairing!!! Now I have to pay for a bumper placement yet once again. what was the use of having no sensors replaced if they were to be replaced at the exact spot you were fixing?