Technically, they shouldn't be accepting Federal Reserve Notes as payment for any state taxes, including sales taxes.
Note that this clause does not authorize the federal government to declare a legal tender or issue paper money; it simply prohibits the states from doing those things. Thanks to the 10th Amendment, those are reserved exclusively to the people. The wording is a bit tortured, but a logical parse is that powers not delegated to the federation are reserved to the states, and that powers prohibited to the states are reserved to the people. Legal tender, and paper money are not delegated to the federation, but prohibited to the states, therefore they remain the province of the people.
There should be no problem with the Federal Reserve printing its own notes, but there's no way in hell that making them legal tender or accepting them for payment on state or federal debts is constitutional. They do it anyway. Hardly anyone cares. Those who do are ignored.
As usual, they will do as they please, damn the consequences, and hang it all up under the interstate commerce clause.
Edit: Article I, section 8, clause 5 does give the federation the power to coin money and standardize its value. It still doesn't give them the power to accept those coins as payment for debts unless they are gold or silver.