I'm not a professional, but let me explain things to you from my (a consumer's) point of view. BTW I've never had my car professionally detailed.
Start rant.
However, my work requires me to go to various hotels, spas, shoe stores, bars, restaurants, etc. where I may be personally served by someone. Often it is difficult to know when and when not to tip, or when it is expected. For example, obviously tips are expected in food related business. Shoe store employees personally tie your shoes, etc, but they work on commission, so it's not expected for the most part. Valets sometimes take tips and sometimes they don't. What if I only parked my car there for 2 minutes, but he opened my door and took car of my car? I don't tip the landscapers or anyone who does work for me around the house. Are they expecting one? I often tip the hairstylist.
End rant.
I guess the rule is that tips are required when the servicer is doing something very personal and custom, and therefore the quality of work can vary greatly according to your idea of satisfaction. Once in a while I'll tip a valet, but I really don't know how they drove the car before it came into my sight. LOL.
Therefore someone may assume the price agreed on is the end of the story. They pay up and send the detailer off. So I wouldn't say it is rude if they didn't know a tip was expected. Now if they KNOW the detailer is expecting a tip, but they don't feel he deserves one, then that's another story. Interpret it as you may, according to the particular situation. Maybe they don't want to spend any more money, so they give no tip or a small one. Maybe they didn't like the service.
I understand tips are consumer's ways of expressing how much they liked your service, but sometimes they are a hassle. It can be embarassing to not know a tip is expected, and then the servicer stands there and lets you know with body language. I think that's unprofessional, and the servicer should just walk away, most of the time. Depends on the situation I guess.
- J