I think it is rather rude and a bit dishonest for people to not divulge their total fees upfront, an of course we should know to ask. But the normal person wouldn't know the questions to ask until they are burnt once like this, and then there still could be costs for walking up stairs or charge another service call if they need to go somewhere to use the bathroom and come back. THAT ISN'T A QUESTION I WOULD THINK TO ASK.
When a person has a blockage in their sewage system they often try to fix it themselves by adding an acid or a base such as Drano in an attempt to dissolve or dislodge the problem. These chemicals can get into the plumbers eyes when the sewage is splashed during the repair. The plumbers skin during the repair does come into contact with the sewage water. The owner of the toilet might not report to the plumber they have already tried Drano a highly caustic base .