Same thing happened to me when I sent a very cheaply priced Sabretooth bust signed by RB and the Shifbros to France for $40. The buyer cried to me about his import taxes demanding that I pay half, which were somehow a total of $40. I basically told him I would not do it, as it was his responsibility and not mine. He came back and told me that he was going to start a PayPal dispute with me saying that the item was never received. Well, he tried, but I provided a copy of his import tax paperwork that he sent me to show the amount of taxes he was to pay, like a friggin' genius!
He ended the dispute, which he would certainly have lost due to the proof, then left me a neg. This punk actually tried to extort money and I said so in the feedback he left for me.
My advice? Tell him to get bent. Take the neg (if he leaves one) and move on with a valuable lesson learned. He's an idiot and it's not your responsibility to pay his taxes. F him! It's the principle involved here. Don't give in.