Normally if you send back something for a warranty repair or replacement it should not have any extra taxes to pay, anywhere in the world.
That's the theory, now in practice, as an individual, every single shipping carrier refuse packages under this kind of procedure and only accept regular expeditions.
Now, anyway, this is an issue for fxtec, you should not advance any money to send back a device under warranty neither receiving it back. And if you do they have to reimburse you the expenses.