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I'm wondering how to open someone in the queue while you're actually at the counter. This happened to me just today and I thought "How could I have opened?" I mean, I was obviously thinking of getting my change and making sure I was given the right amount, but at the same time a nice girl stood right behind me to my right, I looked at her, we smiled and shared glances, then I got my change and left. I thought maybe I was to pragmatic about being a customer and wasn't social enough. Would I wait for her to finish paying for her items and then open? Or 3 second rule? Or kind of start bantering with the clerk, the girl and just be casual about everything? Obviously, I've been in this scenario several times before or I wouldn't ask, anyone care to give me their thoughts on this?

You're simply way overthinking this. If you're looking for a line, it doesn't exist.
Say "Hi. What's your name?" But it's probably more about whether she finds you attractive and things like that.