Here is why:
You will put some items on the counter. A very pleasant, but heavily accented person will say a number to you. This is how much you owe. The problem is, you will not understand what number was said.
If you did not follow my advice in the title, what happens next is and embarrassing repetition of you asking the person working the register to repeat himself, while he becomes increasingly upset that you are not understanding what he is saying.
If you did follow my advice, the solution is simple: you put a $20 bill on the counter. Now watch the person working the register. You will find yourself in one of two states. I have listed the appropriate actions to take depending on which of them you are in:
STATE A: He picks up the $20: all is well.
1) Take your change.
2) Leave with your goods.
STATE B: He frowns or shakes his head.
1) Put another $20 on the counter.
2) Watch the cashier again.
3) Repeat until you move to STATE A.