% and fractions

Customer comes into the store to cash their check.  They get charged and their money counted back to them.  They grab their money and look at me.  Now I know that look and it usually is followed by some sort of a question.  They don't disappoint me, they ask a question.  They ask, "why do my money be different evera time I cash my check?"  I ask them what they mean.  They say that what they receive back is different every time.  I ask them if their check is the same or different every time and they say different.  I then explain that we charge 1.5% of their check to cash it.  They then say that don't explain why their amount is different every time.  Now right about now is the time that I should just let it go and just say that is the way it is and be done with it.  But, I just can't for some reason or another.  I ask them what half of 10 is.  They say, "what do you mean half, we talking bout %?"  I ask what 50% of 10 is and they say 5.  I then ask what 50% of 100 is and they say, "what?"  I put my down on the counter and say, "50."  They say, "50 ain't 50% of 100."  I ask what is 50 + 50?  They ask, "what?"  I then decide that explaining this is futile, but I try one more time anyway.  I tell them that if their check is $125 then we charge 1.5% of that amount but if their check is $225 then we charge 1.5% of that amount and the charge will be bigger.  They just stare at me.  I can see their little mouse running, trying to turn the wheel but it just isn't going to happen.  They just continue to stare and then shrug their shoulders and walks away.  Another customer leaving unsatisfied and confused, but maybe I've lit a spark to help them see that they should...nah, forget it!  They left and my head hurt.  I need to quit trying to explain math to the customers.