Just chatting with Amazon service rep now. All they are willing to do is refund the shipping costs.
Did you make sure that you emphasized this was a charge added after you confirmed the order and that it is not valid for your order date which is prior to the tax being applicable?
I tried to insinuate that it was quite fraudulent to take more funds from a credit card than the shopper agreed to however i did it by asking them to explain why they did this instead of outright accusing them.
I asked if they could explain why additional payment that i did not accept or agree to at checkout was added later with zero feedback to me.
I also asked if they could explain why this tax was being charged to me when it is not applicable to my order which was placed prior to December.
Not sure what approach you used but thats how i did it
After a little more pressure, I received the refund.
I explained that a contract has been formed when I had agreed to the price of the goods that Amazon were supplying by completing the checkout process and that Amazon had confirmed the contract by way of sending the confirmation email with the agreed price.