What you received in the mail was a civil demand letter. Many states have statutes that authorize retailers to collect money from those who shoplifted from their stores.
While the stores have the ability to sue you if you don't pay the demand, many do not enforce their right to collect the money they are entitled to by suing you simply because it would cost the retailers more money in legal fees than they would from you, making it very cost inefficient.
While I don't tell people to ignore the letter, I do tell them to take it to a local criminal defense attorney who can review the letter with you in person and explain to you your obligations with regards to the letter. So, talk to a local lawyer about what you received in the mail before doing anything and do it soon.
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The letter you got in the mail is called a "civil demand." It's a retailer's attempt to convince you to pay them money (a civil "penalty") after you've been accused of shoplifting in their store.
In order to recover any money from you under the Texas Theft Liability Act, the retailer would have to be able to prove that you caused actual money damages to them. In most cases, the store has recovered the merchandise that was allegedly stolen, and there has been no property damage to the store, so there are very few situations where the retailer would have sustained any amount of actual damages that would justify them paying the legal fees they would have to in order to sue you and take you to trial.
Many retailers send threatening demand letters because they're banking on the fact that most people accused of shoplifting aren't aware they don't have to pay the retailers a dime if they cannot prove they suffered actual damages. Your options upon receiving the demand letter case are 1.) to pay what they are asking (probably not advisable, unless you know you caused actual damages to the store), 2.) tell them that you'll see them in court when they file a lawsuit (odds are that they won't waste their money filing suit), or 3.) simply ignore the requests for payment. It's most likely that if you choose options 2 or 3, the retailer will never collect any "civil penalties" from you.
Even more important though, is the fact that if WalMart convinces the police to file a criminal case against you for theft, do NOT plead guilty to the charge unless you have first consulted with an experienced, local defense attorney. Theft is a crime of "moral turpitude," and a conviction for it will have permanent negative effects on your employment ability, etc.