Do Not
Do Not Challenge Returns
If a customer returns a product for any reason whatsoever, take it back and refund their money with a smile. A courteous approach to returns will generate good will and give the customer a reason to shop with you again.
Arguing with the customer will have the opposite effect, they will bad mouth you to everyone they know and that can have a snowball effect that could dramatically affect your business.
The cost of the refund has to weighed up against the greater cost of not refunding. If the message is not clear yet, running a convenience store is all about good will. You must do everything in your power to generate it and then keep it.