I apologize that this question has quite a bit of background!
Many states in the USA have something called a 'sales and use tax'. The basic idea is that whenever you buy something in the state, retailers add on n% to the sticker price of the item, and take that money and send it to the state government (that's the 'sales tax' part). However, sometimes it's possible to buy things without paying sales tax (for instance, some internet purchases are free of sales tax, you might buy things in another state which doesn't have sales tax). In such cases, you have an individual (secular) legal obligation to pay what the sales tax would have been to the state government (that's the 'use tax' part). (If you buy something in a different state that has a lower sales tax rate, you're only obliged to pay the difference between the sales tax you already paid and the sales tax you would have paid in your state.)
A key fact is that the 'use tax' is basically (as far as I can see) only enforced on the honor system (at least if you are not talking about big ticket items like cars). It also seems that the vast majority of Americans do not pay the use tax that they owe.
I, personally, feel obligated to pay this tax as an American, and our family pays it. My wife is willing to humor me on this, but she is a little unhappy about it: she says that she feels like a 'chump' because we're paying this tax that hardly anyone else pays.
It occurred to me that if we have a halachic obligation to pay the tax then maybe my wife will feel better about doing it! Since, in general, taxes are almost the clearest case of dina d'malkuta dina out there, you'd think that that would settle the matter. But many poskim say that dina d'malkuta dina only applies when a law is enforced in practice by the government, which isn't really case for use taxes (at least in our state). So that argument wasn't completely convincing, and it would be nice to find a clear discussion which is exactly on point. Does anyone know of one?
Obviously CYLOR if this actually is a live question for you. As I say, it's not a live question for us because even if we have no halachic obligation to pay this tax, I feel that we must pay for other reasons).