$20 is not enough to get people to go to another state on a $500 purchase. Sales tax is more than that, and there aren't a lot of people buying stuff in other states to avoid that.
Probably not for a lot of the state, but the Charlotte metro area contains a significant number of people who live across the border in Rock Hill for property tax reasons. These people make that commute almost daily. (It's actually enough that the Charlotte Transit Authority has a bus that runs to Rock Hill).