real world diciplinary situations

What do you do next? Scenario #2 The employee who deals with all invoices to customers has missed off the same item on all of the bills for two months. Which means, you have been under-charging. This came to light by chance. You have talked to the employee about the error, told them that accuracy is essential and explained the repercussions - income against what you budgeted to get in is down. Also, explaining to customers that you have been charging the wrong price - and that it will have to go up in future - could cost the business customers. The employee is taking a cavalier attitude to a very serious situation. He admits he has been trained, but can't explain why he made the blunder. What do you do next