I suggest you call the Department of Justice who sent you the letter and explain your situation. Most likely they will be understanding and as long as you are honest with them and not fool/ play around with them, they will agree alternative choice with you (maybe reduce the penalty or allow you to pay a certain amount every month until the penalty is paid in full).
You should not ignore the letter or be dishonest with them otherwise they can take you to court and not only will you need to pay the penalty but you may also have to pay for the court fees which is not cheap!!!! I strongly advise you to contact the person who sent you the letter. They usually include a phone number for you to call them.