I'll note that this is a large part of why I'm opposed to a national sales tax (under any name) replacing the income tax, and I've told more than a few folks this: If you have a 20% tax in that vein (a low estimate by any stretch of the imagination), you're going to get a lot of border-hopping business, especially for things close to but not over the declaration limits (currently $800 per the CBP forms...that'll cover back-to-school shopping pretty easily, for example, and saving $160/trip would be worth the hassle to a lot of folks...especially if they can also fudge the values on things on top of that).
Sales taxes are flat taxes, which means they are mostly regressive in nature. They may look "fair" because everyone pays the exact same rate. However, they are much more punitive to people at the lower rungs of the economic ladder because the relative impact to their purchasing power is far greater. Income tax is at least
intended to be progressive. That is to say it's designed to have less impact on those who are struggling and take more from those who can afford it without serious suffering as a result. There are so many gray areas and loopholes that it doesn't always end up working out that way, but at least there is a bit of reason and compassion being applied to the tax rate. As for me, I would support efforts to reduce or even abandon sales taxes altogether and replacing them with increased income and/or capital gains taxes on a sliding scale rate. For most of the 1980's the US had a top tax bracket of 50%. That sounds absolutely crazy today, but it was just a fact of life back then. In the roaring 20's the top tax bracket was around 75%. In the 1950's the top tax bracket was over 90%. But here in 2012 we're told over and over again that today's 35% top tax rate is far too much of a burden for our wealthiest citizens to pay. It seems to me that the whole trickle-down theory has been proven ineffective. We gave it three decades of continuous trials and watched our economy get weaker and weaker with each boom-bust cycle. At this point I'd like to try something else. Something that isn't based on rewarding wealth or punishing poverty. But maybe that's just me.