Price in basket different from price in description

[moonlight-rhapsody]

I put a book into my basket that said £9.59, free postage. 

The delivery details confirm that although the book is coming from Dublin, it is free postage to the UK. 

When I went to the basket, the price had changed to £11.51 and was still saying free postage. 

How can this be?

[moonlight-rhapsody]

Thank you for the explanation, I didn't know that.