I think that in your case you are very lucky, I don't know you, but from what you say I bet that you have 2 very important things that show off in front of people: you treat people very well and you are attractive, with those gifts you have the work already done believe me. And I think that favored you in that job, and it is what favors many people who work in customer service.

For me, it is okay to offer tips as long as they represent gratitude for a good service received, and of course it is not something that is mandatory to do.