Customer relations!

I'm I wrong?

MOT's are up through the roof of lately with the throughput. This last week we have been working on skeleton staff. Monday I had seven mot's booked in and two members of staff in that day only. The phone is going ten to the dozen and I'm booking mot's in every hour of the day as well as doing the tests. My first mot Monday was a Vauxhall that failed on tyres and brakes. My plan was to wait until lunch if customers did not turn up beforehand to contact them and sort out what they want to do with the repairs. 

My first mot customer rang about mid morning and asked about his mot. I advised that the car had failed on tyres and brakes and that due to workloads we'd not be able to repair his car ourselves Monday. The customer immediately flew up at me on the phone saying that I should not have mot tested his car knowing that I could not do the repairs the same day! I tried to explain that given the car was only booked in for an mot and not service or repairs I could not have known that the car was going to fail. I tried to explain further that we were on skeleton staff and that I had seven mot's to carryout myself that day so I would not be in a position to examine or repair the car. The guy flew up at me on the phone saying that I was patronising him and I stopped talking to him on the phone it was that bad. He came to collect the car with another bloke who was equally as bad as him. He asked for the boss and I said he's on holiday until Monday. The customer went on to say that the boss always compensates him by telling him he can have his car fixed the same day because the boss will fit it in! I tried to explain again that we were on skeleton staff but then he changed his attitude to "I'm not paying for the mot test" until my car is retested and past? I then told him to go to Sainsbury's and buy his weekly shopping and leave the store without paying, and he then replied along the lines of we are a garage and don't have to pay for an mot that has failed and only need to pay when it is retested and past! Anyway I became a bit stern with him and the other guy who was with him told him to pay me the mot fee, which he did. 

Now at the outset on the phone I was very polite and tried explaining our situation but he was not interested, he just shouted at me on the phone implying I'm patronising him!

Did I do something wrong here?