PDC fitment issue

My wife purchased a mazda three 2 years ago from this dealership. We opted for rear pdc's to be installed.However due to the dealership not having enough staff the vehicle was not fetched and not fitted.We found out that the supplier they use was fourways radio. We have had issue previously with them as our vehicles was returned with wrong number plates. We requested a refund to fit the pdc ourselves and was rejected. We requested that instead of PDC we would want smash and grab but was advised that the invoice cannot be changed. We requested to use another supplier and was told that they have already paid the supplier for fitment. How can that be if the work hasn't been carried out. Has any one had issues like this?