After a retail theft. you could receive a civil demand letter from law offices who send them out in bulk and claim the person must pay a fine. That is rarely the case and the money demanded is falsely characterized as a fine.This is rarely true and the money is called a fine; unfortunately, this "fine" is not from the courts and is not truly required.
There are laws in most state which allow you to be sued for the value of the item when there is a retail theft. Most people choose to ignore the demand. This demand is for money over and above any fine you pay if your are ticketed or charged criminally.
You may get a few more letters, each time demanding for more and threatening further legal action, and then they usually stop. While the retailer can sue you, it is extremely rare for anyone to actually be sued. You do not owe any money unless you are sued and the retailer wins. You would have to get notice of any suit and could defend yourself or object to the amount the retailer wants from you.
Do not give into these demands. If you do, you will be in the national retailers database which could affect your ability to get a job and could impact your life in other ways. If you are uncomfortable, hire a lawyer and have the lawyer write a letter. That should be the end of it.