A $2 cup of coffee?

We got the following e-mail from a reader. It is a great example of using Freakonomic thinking to make sense of an otherwise jumbled world:

The reader (who asked to remain anonymous)writes:

Thought I would send an interesting anecdote about incentives:

I go to the Starbucks in my office building twice a day for coffee. I bring a Starbucks insulated cup and most of the clerks get to know me. Apparently, though, even in the world of half-caf, double-shot, soy lattes, with only one pump, my coffee order is confusing. I rarely am charged the same price twice in a row. There is uncertainty, it seems, on the size of the cup I bring and the amount of discount I am supposed to get for bringing it. My best guess is that the proper charge is $1.68, but on any given visit, I am charged anywhere from $1.84 to $1.43.

There is a pattern, however. When a new clerk starts, I usually am charged a price in the higher range by that clerk. After a couple of weeks, the same clerk usually will charge me the right price. This seems reasonable;?cup sizes and discounts that only a few customers get take a couple of weeks to learn. But then, after another week or so, I start getting charged less. Is it my winning smile that has finally won over the clerk? I don’t think so. You see, whatever amount they charge me, I simply dump the change into the tip bin. I figure once they start remembering me as someone who gives his change as a tip, they suddenly underestimate the cup size or find additional discounts for me. To me it’s just a two-dollar cup of coffee every time, but it’s interesting to see the pattern emerge again and again.

Another question worth pondering from a Freakonomics point of view: why does our reader always put the change in the tip bin?