It is impossible to cancel an order

I took my car in for repairs at Mccarthy Volkswagen Silver Oaks yesterday, but even though it was there by 7:30 they could not give me a quote until after 16:00. It then took them till 18:00 to confirm that the parts will only be delivered today in the afternoon. I realised last night that the repairs should rather be an insurance claim and called at 7:00 to cancel the repairs. I could not reach the service adviser until 8:50 and then they said there is no way to cancel the parts, I have to take them, because they are already on the way and therefore belong to me. The parts are standard, how can they possibly not be able to take it back or ever be able to use it again? And how is it possible that the order cannot be cancelled because it is already on the way, but it will only arrive this afternoon. I think this is very bad service. They do not keep any parts in stock and no orders can ever be cancelled. They say if I cancelled in time that would have been fine, but they do not answer phones at night, so how can you possibly cancel? I will definitely never use them again.