Delay for repairs

I took my car for repairs on tuesday the 24th November, on Friday the 27th I called in to check the progress,I was told they are finished with the hail work and my car is in the panelbeating bay.but on the 2nd December I got a message that there is a order for a part that was done and the delivery date will be on the 11 December, my worry is why does it take so long to get the parts and I really need the car as now I'm paying exorbitant amounts for car rentals