If you should have paid it--i.e. it should have been charged to you--you will need to reimburse them. Whenever one person is obligated by law or by contract to pay a certain fee, cost, tax, etc. and another party ends up paying it due to some mistake, that other party would have grounds to sue to recover its money. If you should have paid this sum anyway, then reimbursing the other party puts you in no worse shape than if you'd paid it in the first  place; and since if you don't pay it, you could face legal action--and have legal costs as well as the tax--paying what you are obligated to pay anyway would seem to be the better course.  You could try to fight or refuse their claim; but  if they push it, unless there's some legal or contractual reason you shouldn't pay, you would likely lose.