Your credit card will always be charged in the currency of the lab that are fulfilling your orders through. For example, if you fulfill an order through Loxley Colour which is based in the U.K. you will be charged in £GBP even if your Instaproofs account is based in the U.S.
To see what currency each lab uses please head to the Lab Partners page and click the drop down box at the top of the page.