Excessive charge for garage door service & repair

On 11th April, 2016 Garage door Installations-Gauteng website recommended J Services to repair my automatic garage doors. On arrival there were 4 technicians who arrived and proceeded to investigate. the problem..They then asked when the motors and doors has last been serviced and I sadid a few year ago. They then put forward a proposal for a total of R 6010. ----R1960 service doors-----R2500 service the motors. and sundries of R1550. In my haste I agreed. It took them exactly 20minutes to do the job with a can of Q20 sprayed in to the motor. with an adjustment to one of the springs.. I cannot see how this work warranted this expense. I queried this matter the following day that they had to make a living too. and thereafter was rather rude to me and put down the telephone.