“Fifty-five cents sounds like pocket change — hardly worth worrying about. It was the difference in the sales tax charged in Tennessee and Virginia on recent shopping trips for identical grocery staples like bread and milk. Now consider $254. That’s how much more, on average, Tennessee consumers pay for groceries each year than their counterparts in Virginia, thanks to Tennessee’s 8.225 percent sales tax on food — the highest in the country. Suddenly it seems like more than pocket change. Susan Pruitt hasn’t done the calculations, but she almost always crosses the state line into Virginia to do her grocery shopping.”