Almost every criminal lawyer on this forum has answered this question numerous times. The store cannot fine you. What you received is a civil demand, which is a request for payment, not a fine. If you do pay you harm yourself because then Walmart adds you to the National Retailers database of shoplifters and thieves. Don't pay and nothing will happen. Walmart is not going to court over $250.
I agree that what you received is a "civil demand letter" which is not required to be paid unless a lawsuit is filed. No one on Avvo in the 1 1/2 years of my participation has ever heard of or seen such a lawsuit ever filed. The same analysis applies to the follow up letter(s) you will receive.
The above is general legal and business analysis. It is not "legal advice" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.