If you are looking for a private conversation, the message option is the best way to go. And while tipping is indeed, legally optional, this isn't a legal board. It is pretty clearly established that in the US tipping is part of the social contract that you accept when going out to eat. If I were in a part of the world where the social contract of entering a home included the expectation that I remove my shoes and, knowing this I agreed to enter a home but refused to remove my shoes, I would be ill mannered. I might be excercissing an option open but not all options are polite in all places. So yes, if you enter an establishment knowing the expectation includes tipping for a service, accept that service and then refuse to tip because it is an option open to you then I think you have been ill mannered.