I see this as a great argument for his side, actually. State governments need to be able to enforce taxation at the time of the transaction, because it's going

I see this as a great argument for his side, actually. State governments need to be able to enforce taxation at the time of the transaction, because it's going to be impossible to collect otherwise. Without an effective collection and enforcement mechanism, of course individuals won't go out of their way to make extra payments! Even the tax lawyer arguing the case isn't going out of his way to pay! The question of whether states should be able to charge this tax in the first place is another conversation. But if you accept that levying these taxes is the right choice for society, there needs to be a viable collection mechanism.