Good evening, I'd be happy to help you.
It is fortunate that the store and the police chose not to press charges and simply warned you. As a result, you still have no criminal record and this will not be seen by anyone in the future if your name is ***** ***** the national criminal database.
As for the "civil fine" I assume that this is in the form of a demand letter from the store, looking to recover the costs associated with the theft and loss prevention. It is not unusual to see demands for funds in the area of $400 to $500. This is not a fine, but rather a demand for payment as a punishment. They've likely indicated that they would pursue civil action if it's not paid.
Ultimately, they could sue you, but it's highly unlikely over such a small amount. Since the police have warned you, they are not likely to go back and charge you if the payment is not made. The decision on how to proceed is yours. If you want to put it behind you and not worry about anything else, then you can pay the amount. If you had been charged criminally, you would have incurred costs anyhow to deal with the charge and any penalty.
You've gotten off lightly and are very fortunate. As long as you stay out of trouble from here on out, this will never cause you any further problems.
I look forward to hearing back from you so that we may chat further.