The cashier gave me too much change, and I'm going to keep it.
That sounds selfish. Why not just return the excess change?
It was her dumb mistake, so she will have to pay for it.
Keeping the money from the cashier when you know she gave you too much is stealing and stealing is not right. Give the money back and help the cashier with her mistake.
But she can't count. I know it because she is a girl.
It's wrong to claim that girls can't count as well. Sometimes people get distracted at work and make mistakes. At the end of the day when their registers get counted she could get in trouble and you could have done something to prevented this.