Madness to charge like that! Take it back and demand a full explanation of what was done?
ps when towing (explained in your hand book) you should start off in standard height with the inhibit switch pressed to stop the car lowering, when you get up to speed for long enough, to motorway level. I have had to reset a few cars because this was not done and the car didn't like lowering itself whist pulling something very heavy, horseboxes normally. When towing and the suspension does lower it can upset whatever you are towing. Very interesting article a while ago when someone lent a trailer (something he normally doesn't do) to a chap whose car lowered with a heavy load the trailer took over and both car and trailer were a write off. He definately does not lend his trailer to anyone now!