Grocery Store Is Accusing Me Of Shoplifting At Self Checkout And Sent A Civil Demand Letter. What Can I Do

Location: California. I am really stressed so sorry if this is messy. Last week I went to a big chain grocery store after work and used the self checkout like I always do. I had maybe 15 items, scanned everything, paid with my debit card and walked out. About 10 seconds later a loss prevention guy and a manager stopped me at the door and said the system flagged my transaction as suspicious because a “high value item” was not scanned. It was a pack of allergy meds around 35 dollars that was in the bottom of my basket. I honestly thought I scanned it, but when they replayed the camera in their little office it looks like I moved it past the scanner without the beep. They called it “skip scanning” and said it counts as shoplifting even if it was a mistake. They asked me to sign some form admitting I stole it, give them my ID and pay the price of the item plus a 250 dollar “civil penalty” on the spot or they would call the cops. I got scared, refused to sign anything and said if they think I stole they can just call the police. After about 20 minutes of awkward waiting they let me go but banned me from the store and kept a copy of my ID that they had already taken a picture of. No cops came. Today I got a letter in the mail from some law firm on behalf of the store demanding 375 dollars within 30 days for “civil recovery for theft” and says they “may pursue further legal remedies” if I dont pay. It also says paying does not stop them from going after criminal charges. I have never been in trouble before, I really believed I scanned that item and I even offered to just pay for it at the time. Do I actually have to pay this letter, can they ruin my credit or sue me over this, and should I talk to a local lawyer before I do anything