It's saying that the method of payment you're paying in isn't matching the store method of payment. For example, if the game is listed in Dollars, and you're paying in Euros, it'll convert the total to Euros, and it's going to display the updated total on the next page.
Due to conversion differences, you might end up paying more or less, if it was in Dollars, you'd end up paying less, but as it seems you don't have it, it'll probably be more.