How is the cashier being a crook for herself? If she scans a "secret" item, padding the sale, the extra $30 (in this case) goes into the till, and the computer records it even if the cash never hits the drawer. If the drawer comes up short, the cashier is liable for it. So the cashier cannot be stealing for herself, only the store. Unless I'm missing something, this appears to be fraud perpetuated by the store.
I'm not a lawyer Path, but I do work for them, and I did stay at a Holiday Inn Express once (translation, don't take what I'm about to tell you to court, but do take it seriously)

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First, it is illegal to hold the help accountable for till shortages, shop lifting or dine and dash. You can fire them or have them arrested if they are complicit, but you can't dock their check. Even if you could, you wouldn't because if you do and they get hurt trying to arrest the thief they will sue you to your skivvies.
Second, the store is innocent if the cashier is is trying to use the scanner to cover her expenses. This assuming they did due diligence, and lets pretend they did.
Third, The clerk just bought herself a second charge beyond larceny, and that is conspiracy. What, you say? How is there a conspiracy if Wally World wasn't in on it? Here's the fun part. You can be part of a conspiracy even if your conspirator was unaware of it. In this case, by stealing from Wallmart, and then committing credit card fraud to funnel the proceeds of that larceny to Wallmart (an overt act as an employee) she made Wallmart part of the conspiracy.
Best part? If you live in a state with a three strikes law. you've got them right there. Larceny, credit card fraud, conspiracy. Go directly to jail and do not collect $200.

FQ13