amazon.com - tax refund question

I am in the process of returning an item I bought from US amazon (although I'm in the UK). The package is en route back to the US, but has been acknowledged by amazon.com and I have received a refund to my credit card - but not for the full amount I paid.
My original payment invoice was broken down into three parts - cost of the item, shipping costs and 'est. tax collected' of $10.94. The amount refunded only covers the item and shipping, not the tax element.
How can I go about reclaiming the tax, and if this not levied by amazon, who should I approach?

 

Update, if anyone's interested.
The item was delivered safely back to amazon, so I contacted them to raise the issue. I got a very prompt reply explaining that since the refund had been processed, they could not amend it, but have credited me with $15 against a future purchase (amazon.com only, not marketplace)