The answer is that the question is deceptively asked. You have to be sure that you understand clearly what the situation is. The total amount of money spent is $250 for the box, plus $20 that the clerk kept, for a total of $270. The fans originally spent $300, but got back $10 each, so the total spent is $270 regardless of how you look at it, provided that you look at it clearly.
The closest solution to this question came from John Moser (johnmoser@msn.com). He saw that the question itself was, in fact, quite easily misunderstood. His appetizing solution...
It's a matter of apples and oranges. What do you get when you add 3 apples and 4 oranges? ---- none of the above --- you get 7 fruits. When you compare the wrong things you get an impossible solution. The $270 is the $250 plus the $20 tip(?) therefore adding $20 again creates mass confusion but not a Math problem. Conclusion --- you are ten dollars short because 250 + 20 + 20 = 290 but has nothing to do with the original 300.