Legally, you do not have to show a receipt. If the receipt checker is a police officer, then you have a Fourth Amendment right against a search and seizure
and can refuse the search because he does not have probable cause of shoplifting
. If the receipt checker is a store employee, then he is not a government actor and the Fourth Amendment does not apply. However, when a store employee detains you against your will, they are committing the tort
of false imprisonment
. Store employees have a limited ability to detain you if they have probable cause to believe shoplifting has occurred, under section 490.5 of the Penal Code:
A merchant may detain a person for a reasonable time for
the purpose of conducting an investigation in a reasonable manner
whenever the merchant has probable cause to believe the person to be
detained is attempting to unlawfully take or has unlawfully taken
merchandise from the merchant's premises.
So that protects them from a false imprisonment tort IF they have probable cause. Otherwise, they do not have cause to detain you. You are allowed to refuse, and most stores are aware of that and instruct Loss Prevention not to further pursue an individual if they refuse to have their receipt checked. That's not to say that there will not be a few employees that may give you a hard time, so if you decide to go the route of refusing to allow your receipt to be checked, as a practical matter you have to be prepared to deal with the occasional employee who is ignorant about the law.