try changing:
<option value="American Express" <?php if (!(strcmp("American Express", (isset($_GET["invalid"])?ValidatedField("confirm","Card_Type"):"")))) {echo "selected=\"selected\"";} ?>>American Express</option>
to:
<option value="Amex" <?php if (!(strcmp("Amex", (isset($_GET["invalid"])?ValidatedField("confirm","Card_Type"):"")))) {echo "selected=\"selected\"";} ?>>American Express</option>