An excellent opportunity arises!
You've heard the phrase "You can catch more flies with honey than you can with vinegar"?
The majority of OC encounters in businesses are negative, resulting in the advocated e-mails, letters to corporate, notification of management, etc., etc. complaining about the bad treatment. This often falls on deaf ears and is handled by being completely ignored or (even worse in my opinion) a form letter with the inevitable "We're concerned about your feelings....blah, blah, blah".
It's a rare animal to receive positive attitudes and good treatment by a business. I implore you to do more for the OC cause than any complaint can ever acheive:
Write a letter to the Bank President, corporate, etc. telling them how impressed you are with their service and their attitude regarding your lawfully carried firearm, and how you're going to make them your primary bank and implore your friends to give them their business, as well. (Or, "lay it on thick" if you insist on calling a horse a horse)
This can do more for OC than any negative action will.