I would read the law. My fuzzy recollection is that Texas allows the use of lethal force to stop a burglary or to prevent a thief from escaping with stolen property. IF my recollection on this is correct, a proprietor (or I suppose any customer) could draw down and open fire.
If I were a customer, I don't think I would shoot a thief over a smash-and-grab. Not worth the legal aftermath to protect someone else's property (which is likely insured anyway). If I owned the store ... then I might consider shooting.