I feel that you were able to conduct yourself properly for both parties. As an employee you did your duty in helping out your first customer and acknowledged the other customer and informed him that you would assist him when you were finished. As an Open Carrier, having just conducted yourself properly as a kind employee you were already doing the right thing.
We're always going to get those people who try to rattle us and make us feel bad for carrying a firearm but if we just remain calm and friendly, we win on many levels.