It's rather debatable whether it should be legal for store owners to shoot robbers and call it self-defense; not all states protect shop owners in such circumstances, and state law may differentiate between when robbers pull lethal weaponry and when robbers pull toy guns that were not actual lethal threats. In the US you take further risk if there end up being racial differences between the shop owner and robber. Consider https://www.quora.com/When-is-it-legal-for-a-shop-owner-to-s... as a quickly Googled example.

A shop owner's actual best strategy, in states without firm stand-your-ground or castle doctrine laws that also apply to businesses, is probably an under-the-counter button that calls for police as a silent alarm while responding slowly to stall for time (and consider closing up shop and moving if local police are not quick and reliable to respond). Even in states with more friendly legal environments, risking your life by drawing to defend your inventory or cash register is practically the definition of penny-wise, pound-foolish. You are risking your life over, what, several hundred or a few thousand dollars? And even if you do walk away from the gunfight, how much would it cost to repair all the damage from the gunfight; if you get injured, how much are the hospital bills and subsequent increase in your medical insurance premiums?

No, while the Second Amendment may still be alive on paper, I think its protections don't do much for shop owners these days. A more effective defense would be if that police-alarm button also released a quick-acting sleeping gas, but those aren't really available in real-world contexts and carry lots of unintentional risks.