You always have to read before you sign.
They must disclose that you have currency options.
credit card receipt:
ie: I delcare I have been offered a choice of payment currencies and my choice is final.....
so technically even if you signed the cc receipt. You can dispute the charge if you were not offered the currency charge. You must try to resolve with the merchant, but a dispute pulls the money out of the merhcant's account
and excessive chargebacks increase their cc proccessing rate and can lead to
canceleing the merchant account.
So check your receipt. If you were not verbally offered currency choice than that is a breach of the merchant's agreement with the bank credit card processing.