The cashier gave me extra change and I kept it.
Why wouldn't you give the cashier back the extra money they accidently gave you?
I'd rather keep it for myself
Sometimes cashiers make mistakes, but customers shouldn't intentionally take advantage of these mistakes. It's expected that someone fix an accident if it is in their power to do so. The cashier may get in trouble when their money totals at night don't match up with what their machine says it should be.
Nah I'm going to keep the money.
Please return the money if it is more than a dollar. She might be on the hook to pay it back out of her paycheck or she could be accused of pocketing the money.