is liable to the United States for a civil penalty of not less than $500 for each violation and for each additional day the violation continues; except that, in the case of a person who is not registered under this part to provide transportation of passengers, or an officer, agent, or employee of such person, that does not comply with section
13901 with respect to providing transportation of passengers, the amount of the civil penalty shall not be less than $2,000 for each violation and for each additional day the violation continues.