Mark's suggestion is certainly one way to go about things. I would probably proceed differently. I would attempt to educate the officer as to the legality of open carry. Now, if the store manager asks him to deal with it then you have only 2 options...you can leave or you can go to your car and remove your weapon. It's that simple. But it's very unlikely that an officer will outright arrest you for OC as there is no law being broken. If the officer says you're under arrest and have the right to remain silent before even speaking or asking you a question, then remain silent and get an attorney.
Simply put, try to educate him as best you can, give him a LOCAL pamphlet if you have one, and if he insists that you disarm you either leave and shop elsewhere or do as he says.
In my experience TPSO has some pretty good folks working there so you are unlikely to have much of a problem. I've OCed in other China Marts before and haven't had a problem so far.