To be clear, the $400 includes fees, e.g. charges imposed by the carrier and perhaps FlyEx for all we know. There should be an available breakdown of what the taxes are. In any event, you are seeking a refund of both, not just the taxes.
As noted, should not matter, but the better informed you are, the better you will fare in a chargeback as merchant vendors spend a lot of time these days pushing back and it is easy enough to lose out if you are not fully prepared.