When I arrange for my car to be serviced, they tell me exactly what will be done during the service and how much it will cost. They also advise me that if any consumable items are worn out, I will be charged extra for replacements but only after they have confirmed with me that it is OK for them to do this. If they come across any problems during the service, they make a note of them to discuss with me later unless it is something really urgent or dangerous, in which case they will try to contact me immediately and even put the work on hold if necessary. After the service, along with the bill, they will inform me of any problems they've found, advise me of the urgency of each one and give me a breakdown of the cost of repairing each of them. All their prices are prominently displayed in the office and work area. I am never made to feel obliged to accept their proposals and they make it clear they are happy for me to go elsewhere if I am not happy with their terms.
On the other hand, I have heard stories on consumer affairs programmes of customers that have left vehicles with unscrupulous garages and then been presented with ludicrous bills when they arrived to collect them. In these cases, the tradesman usually gives the excuse that, in his expert opinion, the work needed doing and so he went ahead and did it without consulting the vehicle's owner, usually claiming it to be a so-called favour. You can find examples of this type of behaviour in many industries and, without exception, the perpetrators are considered shady, if not criminal. No one in their right mind believes any of their excuses and the tradesmen concerned is often labelled a cowboy.
Software developers also try similar tactics. "I can't give you an estimate up front. I won't know how long it will take until I've actually done the work." Or, "I can tell you how long it will take to do the whole job but I can't break it down or itemise it". I find it very interesting that software developers try to defend such behaviour by labelling it 'professionalism', but in other industries it's considered just the opposite.
So when I hear the cry, "but I'm a professional", I'm about as convinced the proclaimer really is a professional as I am when I hear Mr Walliams declare, "but I'm a lady" on the popular TV sketch show. I only wish I were as amused.®