I see Upchurch arguing general concepts and facts and you arguing your own personal perspective. That's ok as far as it goes, namely just you, but otherwise Upchurch is wiping the floor with you.
To the extent that's true, it's because he is arguing against a straw man.
Is the flag a symbol of racism? Of course it is, to an awful lot of people, including supporters and detractors of the flag. There's no argument there, so any attempt to argue against that is just arguing against a straw man.
So, now, restate an argument without reference to that position, and we'll see if the argument makes any sense.
My argument is that it is, among other things, an historical symbol. It is appropriate to sell it in a place that commemorates the history associated with that symbol.
Another aspect of my argument is that when the sale of a product is restricted by private parties for fear of economic reprisal, then freedom of expression has been restricted. The Libertarian Party would disagree. Only government can restrict free expression, according to the Libertarian philosophy. I disagree with them.
One other element that has come up is the people who are saying the flag represents "heritage, not hate". Some people, specifically including Upchurch, have denounced that position. I believe that people who do so would say that the heritage of that flag IS hate, so the position makes no sense. I don't disagree with that reasoning, but I think it misses an important point. When someone defends the flag by saying "heritage, not hate", what you have is a Confederate flag waver who is denouncing racism. That's a win. You should see it that way.