I had something similar happen when i carried into a gas station in Frankfort the other day the poor guy seemed like he was going to just pass out lol, I was in nice khakis and a button up short sleeve shirt tucked in etc with my xdm on my side!
However I do wish i had the great idea of the business card as you do, maybe shoot me a message with the format or template you used?! I dont think im creative enough! I did of course inform the gentleman I was simply carrying for mine and his protection.
I think its awesome you made sure to inform them and pass along the education! Lets hope he takes it as a learning curve thoe and doesn't expect the next guy who talks in armed to be as kind! ( unless of course the guy has it holstered and wearing a suit and tie?!)