Many states (Arizona and Ohio come to mind from personal experience) allow counties to tack on a local surcharge, making the sales tax rate vary by county. Allegheny (Pittsburgh) and Philadelphia counties in Pennsylvania each charge an extra 1%, and the rules in Pennsylvania for what item are and are not subject to sales tax can be tricky to understand. Some states levy different tax rates on different sorts of consumer goods.

Yeah, it might make sense to bury the details in a module, where all the really hairy bits can get hidden, but then you have to maintain that sucker to keep up with changes in the tax laws. It's not pretty.

I suspect that things are a bit simpler in Europe in general and Belgium in particular.