Is an employee sinful if he follows the official and costly method for sending correspondence (registered mail with acknowledgment of receipt) instead of a less costly and unofficial method that would expose him to administrative accountability?

The reference for the permissibility or impermissibility of following the second, more expensive method for the company is the company itself, because it is the owner of the right and the authority. The employee is an agent who acts only within the limits of what he has been authorized to do. And customary permission is like verbal permission according to the scholars.