In most cases, a receipt check is voluntary, but several wholesale shopping clubs make you agree to them as part of the membership contract.

Thanks to everyone who helped unearth these. No shoplifting mentioned. Their given reasons for checking the receipts don't make any sense. If all the checker does it glance and the receipt and make a highlighter stroke across it, how is he checking the correct price?

Sam's Club: "To ensure that you are charged correctly for the merchandise you have selected, you will be requested to show your receipt when exiting."

Costco [PDF, pg 14]: "To ensure that all members are correctly charged for the merchandise purchased, all receipts and merchandise will be inspected as you leave the warehouse."

BJ's Wholesale Club: "BJ's inspects all receipts as Members exit the Club to ensure that Members have not been overcharged or undercharged for an item and that all selections appear on the receipt."

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Adventures In Receipt Check Refusals Continue
Circuit City Customer Arrested After Refusing To Show Receipt
TigerDirect Apologizes For Unlawfully Detaining Customer For Refusing To Show Receipt
TigerDirect Unlawfully Restrains And Verbally Abuses Customer For Not Submitting To Receipt-Showing Demands