Problem with GameGears, need your opinion

[Queen of Mudcrabs]

Today I received an email from GameGears about an error they made when they calculated the shipping costs for my order.

Here is the email:

Hi [my name],

Hello and thank you fro buying from Gamegears, Upon processing your order it was brough to our attention that the postage that you paid for the item was incorrect. The postage cost should have been £12GBP. Because of this you will need to pay an extra £10.05GBP. Please let me know if you would like for us yto invoice you so that you can make the payment. Please accept our apologies for any inconvenience this may have caused and I look forward to hearing from you soon. Regards, Georgina care@gamegears.co.uk

Thank you,
GameGears

I really do not want to pay 10 extra pounds for a mistake they made. Plus, if I knew that shipping would cost me £12 I would not have ordered from them in the first place.
I can't be held responsible for their mistake so legally I am not obliged to pay the extra money, right?