Every major store hires one of these law firms to send out civil demand letters to shoplifters. The demand amount is usually the maximum the law in your state allows as a penalty that could be imposed in addition to the actual value of the item(s) taken (if not returned) and court costs and sometimes attorneys fees. I've never heard of the firm your letter came from, but these letters are rarely (if ever) acted on. It is only profitable if people simply pay the demand, it is usually too costly for them to actually sue.
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